Massmutual Premier Funds N-CSRCertified annual shareholder report (management investment company)

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM N-CSR

CERTIFIED SHAREHOLDER REPORT OF REGISTERED

MANAGEMENT INVESTMENT COMPANIES

Item 1. Reports to Stockholders.

This material must be preceded or accompanied by a current prospectus (or summary prospectus, if available) for the MassMutual Premier Funds. Investors should consider a Fund’s investment objective, risks, and charges and expenses carefully before investing. This and other information about the investment company is available in the prospectus (or summary prospectus, if available). Read it carefully before investing.

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To Our Shareholders

“MassMutual recognizes that talking about your future can be uncomfortable. That’s why we’re here – to help you build the future you want for yourself and your family. For nearly 170 years, through ups and downs and good times and bad, we have helped our clients protect their loved ones and prepare for a better tomorrow.”

September 30, 2021

The return of market volatility challenges retirement investors

I am pleased to present you with the MassMutual Premier Funds Annual Report. During the fiscal year ended September 30, 2021, U.S. stocks were up over 30%, with investors seeing an end in sight to the global economic recession brought on by the COVID-19 pandemic. Foreign stocks in developed markets and emerging markets also experienced strong gains in the fiscal year, aided by increasing economic activity, generous fiscal and monetary stimulus, and strong corporate earnings throughout the reporting period. U.S. bond investors experienced negative returns in the rising interest rate environment, fueled by an increase in economic growth expectations, heightened inflationary pressures, and a lessening of monetary and fiscal policy support.

The return of volatility and the reality of market sell-offs can test an investor’s mettle, but also serve to remind investors that it’s important to maintain perspective and have realistic expectations about the future performance of your investment accounts. As described in this report, financial markets can reverse suddenly with little or no notice. That’s why we continue to believe that retirement investors should create and maintain a plan that focuses on their goals, how long they have to invest, and how comfortable they are with market volatility.

Suggestions for retirement investors under any market conditions

*Systematic investing and asset allocation do not ensure a profit or protect against loss in a declining market. Systematic investing involves continuous investment in securities regardless of fluctuating price levels. Investors should consider their ability to continue investing through periods of low price levels.

(Continued)

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Is it time to track your progress?

If you work with a financial professional, you may wish to consider checking in with him or her to help you determine if:

Get to where you want to be

MassMutual recognizes that talking about your future can be uncomfortable. That’s why we’re here – to help you build the future you want for yourself and your family. For nearly 170 years, through ups and downs and good times and bad, we have helped our clients protect their loved ones and prepare for a better tomorrow. So we continue to encourage you to maintain perspective when it comes to retirement investing, regardless of any short-term changes in the markets or headline-making events that may be unsettling. MassMutual’s view is that changing market conditions have the potential to reward patient investors. Thank you for your continued confidence and trust in MassMutual.

Sincerely,

Paul LaPiana
President

The information provided is the opinion of MML Investment Advisers, LLC (MML Advisers) and is subject to change without notice. It is not to be construed as tax, legal, or investment advice. Of course, past performance does not guarantee future results.

^© 2021 Massachusetts Mutual Life Insurance Company (MassMutual^®), Springfield, MA 01111-0001. All rights reserved. www.MassMutual.com Underwriter: MML Distributors, LLC. (MMLD) Member FINRA and SIPC (www.FINRA.org and www.SIPC.org), 1295 State Street, Springfield, MA 01111-0001. MMLD is a wholly-owned subsidiary of MassMutual. Investment advisory services provided to the Funds by MML Advisers, a wholly-owned subsidiary of MassMutual.

RS-50676-00

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September 30, 2021

Market Highlights

Market Environment

Global stock investors experienced declining levels of volatility and strong positive returns for the fiscal year beginning October 1, 2020. U.S. stocks rose steadily in the period, buoyed by the Federal Reserve Board’s commitment to low interest rates, additional rounds of fiscal stimulus, rising expectations for strong economic growth, and rebounding corporate earnings. As a result, the broad market S&P 500^® Index* (the “S&P 500”) delivered a strong 30.00% return for the fiscal year. The Dow Jones Industrial Average^SM was also up a respectable 24.15%. The technology-heavy NASDAQ Composite Index was also a big winner, advancing 30.26% for the fiscal year. Small- and mid-cap stocks outperformed their larger peers, as small-caps gained 47.68% – while value stocks outperformed their growth counterparts by approximately 8% during the period.

The continued market recovery, additional fiscal stimulus, conclusion of the U.S. presidential election, expectations for the tightening of monetary policy, and heightened inflationary pressures affected sectors differently, though all 11 sectors were in positive territory for the fiscal year. The energy, financial, and communication services sectors led in performance, enjoying double-digit growth, as they outperformed the S&P 500. The energy sector was the strongest performer for the fiscal year, up over 80%, aided by an 87% increase in the price of West Texas Intermediate (WTI) crude oil during the period. The utilities and consumer staples sectors trailed the S&P 500, each by nearly 20%.

Developed international markets, as measured by the MSCI EAFE^® Index, trailed their domestic peers, ending the fiscal year up 25.73%. Emerging-market stocks, as measured by the MSCI Emerging Markets Index, ended up 18.20% for the period. Developed international and emerging-market stocks benefited from increasing economic activity, generous fiscal and monetary stimulus, and strong corporate earnings during the fiscal year.

Bond yields rose sharply during the fiscal year, with the 10-year U.S. Treasury bond yield rising from a low of 0.68% on October 1, 2020 to close the period at 1.52%. Rising yields generally produce falling bond prices; consequently, bond index returns suffered. The Bloomberg U.S. Aggregate Bond Index ended the period down 0.90%. Investment-grade corporate bonds fared

* Indexes referenced, other than the MSCI Indexes, are unmanaged, do not incur fees, expenses, or taxes, and cannot be purchased directly for investment. The MSCI Indexes are unmanaged, do not incur fees or expenses, and cannot be purchased directly for investment.

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somewhat better. The Bloomberg U.S. Corporate Bond Index, which tracks investment-grade corporate bonds, ended the period with a 1.74% gain. The Bloomberg U.S. Corporate High Yield Bond Index also ended in positive territory, advancing 11.28%, aided by a sharp increase in commodity prices.

Review and maintain your strategy

MassMutual is committed to helping people secure their financial future and protect the ones they love. While the return of volatility and the reality of market sell-offs can test an investor’s mettle, we’d like to remind you as a retirement investor that it’s important to maintain perspective and have realistic expectations about the future performance of your investment accounts. As described in this report, financial markets can reverse suddenly with little or no notice. Our multi-managed and sub-advised mutual funds tap into the deep expertise of seasoned asset managers who are committed to helping long-term investors prepare for retirement – in all market conditions. As always, we recommend that you work with a personal financial professional, who can help you define an investment strategy that aligns with your comfort level with respect to market volatility, how long you have to save and invest, and your specific financial goals. Thank you for your confidence in MassMutual.

^© 2021 Massachusetts Mutual Life Insurance Company (MassMutual^®), Springfield, MA 01111-0001. All rights reserved. www.MassMutual.com Underwriter: MML Distributors, LLC. (MMLD) Member FINRA and SIPC (www.FINRA.org and www.SIPC.org), 1295 State Street, Springfield, MA 01111-0001. MMLD is a wholly-owned subsidiary of MassMutual. Investment advisory services provided to the Funds by MML Investment Advisers, LLC (MML Advisers), a wholly-owned subsidiary of MassMutual. The information provided is the opinion of MML Advisers as of 10/1/21 and is subject to change without notice. It is not to be construed as tax, legal, or investment advice. Of course, past performance does not guarantee future results.

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What is the investment approach of MassMutual U.S. Government Money Market Fund*, and who is the Fund’s subadviser?

The Fund seeks current income consistent with preservation of capital and liquidity. The Fund normally invests at least 99.5% of its total assets in cash, U.S. Government securities, and/or repurchase agreements fully collateralized by cash or U.S. Government securities. Under normal circumstances, the Fund invests at least 80% of its net assets in U.S. Government securities and repurchase agreements that are fully collateralized by U.S. Government securities. The Fund’s subadviser is Barings LLC (Barings). The Fund seeks to maintain, but does not guarantee, a stable $1.00 share price. You could lose money by investing in the Fund. Although the Fund seeks to preserve the value of your investment at $1.00 per share, it cannot guarantee it will do so. An investment in the Fund is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. The Fund’s sponsor has no legal obligation to provide financial support to the Fund, and you should not expect that the sponsor will provide financial support to the Fund at any time.

How did the Fund perform during the 12 months ended September 30, 2021?

The Fund’s Class R5 shares returned 0.00%, underperforming the 0.06% return of the FTSE 3-Month Treasury Bill Index (the “benchmark”), which measures daily performance of 3-month US Treasury bills.

For a discussion on the economic and market environment during the 12-month period ended September 30, 2021, please see the Economic and Market Overview, beginning on page 3.

Subadviser discussion of factors that contributed to the Fund’s performance

The U.S. Federal Reserve (the Fed) held rates steady over the course of the year, remaining firm at a target range of 0.00% to 0.25% as the economic recovery continued to take hold. While there is speculation as to when the Fed will begin to taper its asset purchases, market participants believe the Fed is signaling that tapering is possible sometime in the fourth quarter of 2021 or possibly the first quarter of 2022. LIBOR (London Interbank Offered Rate) rates decreased, with 1-month LIBOR down 0.07% to 0.07% over the year, and 3-month LIBOR down 0.10% to 0.13% over the 12-month period. Both 1-month and 3-month LIBOR hit all-time record lows in 2021, as demand far exceeded supply throughout the year. Secured Overnight Financing Rates (SOFR) briefly got above 0.10%, but spent almost the entire year in mid-single digits, even getting down to 0.01% for three months earlier in the year. The Fed made a technical adjustment at its June meeting, which pushed SOFR up to 0.05%, where it has stayed since then.

At period-end, Government money market fund balances were still at very high levels. This, in tandem with the decrease in net Treasury Bill supply in 2021, made finding any real yield difficult, as too much cash has been chasing too few assets. The decrease in SOFR earlier in the year also had a negative effect on the Fund’s performance due to the number of floating-rate bonds in the Fund that were tied to SOFR. As of September 30, 2021, the yield curve out to one year was extremely flat, with no expected rate hike in the next 12 months. Consequently, Fund management concluded that there was little incentive to extend the duration of the Fund at that time. (The yield curve is a graph showing the term structure of interest rates by plotting the yields of all bonds of comparable quality with maturities ranging from the shortest – typically one month – to the longest – typically 30 years – available. The resulting curve shows whether short-term interest rates are higher or lower than long-term rates. Duration is a measure of a bond fund’s sensitivity to interest rates. The longer the duration, the greater the price impact on the bond or portfolio when interest rates rise or fall.)

Subadviser outlook

Fund management believes that market trends finished the period showing strong signs of improvement. Minutes released from the Fed’s latest meeting confirmed that the Federal Open Market Committee (FOMC) will continue its asset purchases as the path of economic recovery continues to be dictated by the direction and trends of the COVID-19 virus. Nevertheless, Fed Chair Powell reiterated the Fed’s commitment to its dual mandate of promoting maximum employment and fostering price stability, while also noting that economic indicators point to progress on both fronts. Moving toward 2022, Barings believes that the Fed will prudently approach the market concern over inflation, and act appropriately as economic indicators continue to shape the

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“reopening” story. Fund management plans to monitor Fed language carefully and prepare accordingly if signs of rate hikes emerge. While there are still unknowns leading into the fourth quarter of 2021, Barings plans to maintain a defensive stance, while remaining diligent to its fundamental bottom-up process of capital preservation.

6

GROWTH OF $10,000 INVESTMENT - Class R5

The graph above illustrates the Fund’s historical performance for the past 10 fiscal years or since inception (for Funds lacking 10-year records) in comparison to its benchmark index, as well as one or more additional indexes, if applicable.

Generally accepted accounting principles require adjustments to be made to the net assets of the Fund at period end for financial reporting purposes only, and as such, the total return based on the unadjusted net asset value per share may differ from the total return reported in the financial highlights.

Performance results in the graph and table are shown both with and without the imposition of the maximum applicable sales charge (if applicable) and reflect the percentage change in net asset value, including reinvestment of dividends and capital gains distributions. Investors should note that the Fund is a professionally managed mutual fund, while the index or indexes shown above are unmanaged, cannot be purchased directly, and, with the exception of any peer group index, do not incur expenses.

Performance data quoted in the graph and table represents past performance; past performance is not predictive of future results. The investment return and principal value of shares of the Fund fluctuate, so your shares, when sold, may be worth more or less than their original cost. Current performance of the Fund may be lower or higher than the performance quoted. The performance shown does not reflect the deduction of taxes, if any, that a shareholder would pay on Fund distributions or the redemption of Fund shares. Performance data current to the most recent month end may be obtained by calling 1-888-309-3539.

Investors should read the Fund’s prospectus with regard to the Fund’s investment objectives, risks, and charges and expenses in conjunction with these financial statements. Employee retirement benefit plans that invest plan assets in the Separate Investment Accounts (SIAs) may be subject to certain charges as set forth in their respective Plan Documents. Total return figures would be lower for the periods presented if they reflected these charges.

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What is the investment approach of MassMutual Short-Duration Bond Fund*, and who is the Fund’s subadviser?

The Fund seeks to achieve a high total rate of return primarily from current income while minimizing fluctuations in capital values by investing primarily in a diversified portfolio of short-term investment grade fixed income securities. Under normal circumstances, the Fund invests at least 80% of its net assets (plus the amount of any borrowings for investment purposes) in investment grade fixed income securities (rated Baa3 or higher by Moody’s, BBB- or higher by Standard & Poor’s or the equivalent by any nationally recognized statistical rating organization, or, if unrated, determined to be of comparable quality by the subadviser). The Fund’s subadviser is Barings LLC (Barings). Effective May 1, 2021, Baring International Investment Limited was added as sub-subadviser to the Fund.

How did the Fund perform during the 12 months ended September 30, 2021?

The Fund’s Class R5 shares returned 4.56%, outperforming the 0.29% return of the Bloomberg U.S. Aggregate 1-3 Year Bond Index (the “benchmark”), which measures the performance of investment grade, U.S. dollar-denominated, fixed-rate taxable bond market securities with maturities of 1-3 years, including Treasuries, government-related and corporate securities, mortgage-backed securities (MBS) (agency fixed-rate and hybrid ARM pass-throughs), asset-backed securities (ABS), and commercial mortgage-backed securities (CMBS). (“Agency” refers to debt issued by U.S. federal government agencies or government-sponsored entities for financing purposes.) The Fund outperformed the 0.03% return of the Bloomberg U.S. 1-3 Year Government Bond Index, which measures the performance of the U.S. Treasury and U.S. Agency Indexes with maturities of 1-3 years, including Treasuries and U.S. agency debentures. It is a component of the U.S. Government/Credit Index and the U.S. Aggregate Index. Going forward, the Fund’s performance benchmark index will be the Bloomberg U.S. Aggregate 1-3 Year Bond Index rather than the Bloomberg U.S. 1-3 Year Government Bond Index because the Bloomberg U.S. Aggregate 1-3 Year Bond Index more closely represents the Fund’s investment strategy.

For a discussion on the economic and market environment during the 12-month period ended September 30, 2021, please see the Economic and Market Overview, beginning on page 3.

Subadviser discussion of factors that contributed to the Fund’s performance

Over the course of the year ended September 30, 2021, the U.S. Treasury curve between 3 months and 3 years remained flat, and as a result, the Fund held a shorter duration positioning position relative to the benchmark. (The yield curve is a graph showing the term structure of interest rates by plotting the yields of all bonds of comparable quality with maturities ranging from the shortest – typically one month – to the longest – typically 30 years – available. The resulting curve shows whether short-term interest rates are higher or lower than long-term rates. Duration is a measure of a bond fund’s sensitivity to interest rates. The longer the duration, the greater the price impact on the bond or portfolio when interest rates rise or fall.) Bond duration and maturity positioning contributed to the Fund’s performance over the last 12 months as front-end rates increased. At period-end, the duration of the Fund stood at 0.46 years. An underweight allocation to U.S. Treasuries detracted from Fund performance.

The Fund’s allocation to corporate credit was a top contributor to returns. Investment-grade and BB-rated high yield holdings contributed positively. Fund holdings in finance-other, secondary oil & gas producers, and leasing were also among the top-performing industry holdings. On the other hand, food processors and airlines were among the top-detracting industry holdings. The Fund continues to favor industries less susceptible to merger and acquisition (M&A) risk, such as banking and diversified industrial industries.

ABS was also a top contributor to performance during the reporting period, as the Fund held an overweight allocation to the sector. The asset class underwent significant spread tightening over the period as a result of improving consumer fundamentals heading out of the COVID-19 pandemic. The consumer and commercial ABS markets had the tailwind of strong and improving fundamentals giving momentum to tighter spreads across several collateral subsectors. This theme has been consistent for the past several months. CMBS and residential mortgage-backed securities (RMBS) both contributed positively to performance over the period. On the other hand, the municipal and MBS allocations detracted from returns.

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At period-end, the Fund remained overweight in sectors such as corporates and securitized relative to liquid products, such as governments and agencies.

The Fund uses derivative instruments on occasion for yield curve, duration, downside risk management and to gain exposures. Derivatives are securities that derive their value from the performance of one or more other investments and take the form of a contract between two or more parties. Most derivatives are used for hedging, speculation, or both. They may also be used as a substitute for a direct investment. Performance from derivatives contributed negatively to performance over the last year.

Subadviser outlook

Market trends finished the period showing strong signs of improvement. Minutes released from the Federal Reserve’s (the Fed) latest meeting confirmed that the Federal Open Market Committee (FOMC) will continue its asset purchases as the path of economic recovery continues to be dictated by the direction and trends of the COVID-19 virus. Nevertheless, Fed Chair Powell reiterated the Fed’s commitment to its dual mandate of promoting maximum employment and fostering price stability, while also noting that economic indicators point to progress on both fronts. As we move forward, Barings believes that the Fed will prudently approach the market concern over inflation, and act appropriately as economic indicators continue to shape the “reopening” story. While there are still unknowns leading into the fourth quarter of 2021, Fund management plans to maintain a defensive stance, while remaining diligent to Barings’ fundamental bottom-up process of capital preservation.

9

GROWTH OF $10,000 INVESTMENT - Class R5

The graph above illustrates a representative class of the Fund’s historical performance for the past 10 fiscal years or since inception (for Funds lacking 10-year records) in comparison to its benchmark index, as well as one or more additional indexes, if applicable. The performance of other share classes will be greater than or less than the class depicted above.

Generally accepted accounting principles require adjustments to be made to the net assets of the Fund at period end for financial reporting purposes only, and as such, the total return based on the unadjusted net asset value per share may differ from the total return reported in the financial highlights.

Performance results in the graph and table are shown both with and without the imposition of the maximum applicable sales charge (if applicable) and reflect the percentage change in net asset value, including reinvestment of dividends and capital gains distributions. Investors should note that the Fund is a professionally managed mutual fund, while the index or indexes shown above are unmanaged, cannot be purchased directly, and, with the exception of any peer group index, do not incur expenses.

Performance data quoted in the graph and table represents past performance; past performance is not predictive of future results. The investment return and principal value of shares of the Fund fluctuate, so your shares, when sold, may be worth more or less than their original cost. Current performance of the Fund may be lower or higher than the performance quoted. The performance shown does not reflect the deduction of taxes, if any, that a shareholder would pay on Fund distributions or the redemption of Fund shares. Performance data current to the most recent month end may be obtained by calling 1-888-309-3539.

Investors should read the Fund’s prospectus with regard to the Fund’s investment objectives, risks, and charges and expenses in conjunction with these financial statements. Employee retirement benefit plans that invest plan assets in the Separate Investment Accounts (SIAs) may be subject to certain charges as set forth in their respective Plan Documents. Total return figures would be lower for the periods presented if they reflected these charges.

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What is the investment approach of MassMutual Inflation-Protected and Income Fund*, and who is the Fund’s subadviser?

The Fund seeks to achieve as high a total rate of real return on an annual basis as is considered consistent with prudent investment risk and the preservation of capital by investing, under normal circumstances, at least 80% of its net assets (plus the amount of any borrowings for investment purposes) in inflation-indexed bonds and other income-producing securities. The Fund’s subadviser is Barings LLC (Barings). Effective May 1, 2021, Baring International Investment Limited was added as sub-subadviser to the Fund.

How did the Fund perform during the 12 months ended September 30, 2021?

The Fund’s Class R5 shares returned 6.01%, outperforming the 5.19% return of the Bloomberg U.S. Treasury Inflation Protected Securities (TIPS) Index (Series-L) (the “benchmark”), which measures the performance of rules-based, market value-weighted inflation-protected securities issued by the U.S. Treasury. It is a subset of the Global Inflation-Linked Index (Series-L).

How do inflation-indexed bonds protect against inflation?

Like many other fixed-income securities, inflation-indexed bonds pay income twice a year, based on a stated coupon rate. However, both the principal and the interest payment are adjusted for the level of inflation. The inflation rate – as measured by the Consumer Price Index for All Urban Consumers (“CPI-U”) – results in an adjustment to the principal amount of an inflation-protected security. The coupon rate is then applied to the adjusted principal amount to determine the interest payment. For example, assuming an inflation rate of 3%, a security with a par value of $1,000 and an annual coupon rate of 1.75%, the adjusted principal amount after one year would be $1,030 ($1,000 increased by 3%). The semi-annual interest payment would be calculated by multiplying $1,030 by 1.75% and dividing by 2 instead of using the original $1,000 par value to calculate the amount of interest.

For a discussion on the economic and market environment during the 12-month period ended September 30, 2021, please see the Economic and Market Overview, beginning on page 3.

Subadviser discussion of factors that contributed to the Fund’s performance

Consumer prices, as measured by the U.S. Bureau of Labor Statistics, were up for the year-to-date 2021, as unadjusted Headline CPI came in at 5.3% for the month of August, up from 1.4% at year-end 2020. (The Consumer Price Index [CPI] measures changes in the price of a market basket of consumer goods and services; Core CPI does not include food and energy prices. Headline CPI includes food and energy.) Core CPI increased to 4.0% for the month of August, up from 1.6% at year-end 2020. Core CPI is running well above the 2% inflation objective of the Federal Open Market Committee (FOMC).

TIPS positioning in aggregate detracted from Fund performance. A small allocation to high-quality income-producing assets contributed to performance during the reporting period. In addition to TIPS, the Fund also invested in high-quality income-producing securities, including asset-backed and money market securities. The income earned by these asset classes contributed positively over the last 12 months. Allocations to asset-backed securities (ABS) was the main driver of performance. Security selections in auto loans and student loans within the ABS sector were the primary contributors.

The Fund may use derivative instruments for yield curve and duration management, as well as for replicating cash bonds and gaining market exposure in order to implement manager views. (Derivatives are securities that derive their value from the performance of one or more other investments and take the form of a contract between two or more parties. Derivatives can be used for hedging, speculation, or both.) During the fiscal year, derivatives detracted marginally from the Fund’s performance.

Subadviser outlook

While monetary and fiscal programs as well as positive vaccination news have bolstered inflation expectations, Barings continues to believe much of this bullish inflation sentiment has largely been priced into the inflation markets. After the June FOMC and a new hawkish tone regarding tapering, some weakness did emerge in the inflation markets. However, the still dovish Federal

11

Reserve, positive vaccination news, and the reopening of global economies led the Barings team to take a constructive, but more balanced, view on the potential impact to inflation markets. Acknowledging short-term inflation fundamentals may register as elevated throughout 2021, Fund management believes that valuations may have peaked and could remain well anchored during the fourth quarter of 2021 and into 2022. Fund Management’s belief is based on perceived employment and growth trends for the U.S. economy as well as seasonal price pressures that tend to be more pronounced in the winter months.

12

GROWTH OF $10,000 INVESTMENT - Class R5

The graph above illustrates a representative class of the Fund’s historical performance for the past 10 fiscal years or since inception (for Funds lacking 10-year records) in comparison to its benchmark index, as well as one or more additional indexes, if applicable. The performance of other share classes will be greater than or less than the class depicted above.

Generally accepted accounting principles require adjustments to be made to the net assets of the Fund at period end for financial reporting purposes only, and as such, the total return based on the unadjusted net asset value per share may differ from the total return reported in the financial highlights.

Performance results in the graph and table are shown both with and without the imposition of the maximum applicable sales charge (if applicable) and reflect the percentage change in net asset value, including reinvestment of dividends and capital gains distributions. Investors should note that the Fund is a professionally managed mutual fund, while the index or indexes shown above are unmanaged, cannot be purchased directly, and, with the exception of any peer group index, do not incur expenses.

Performance data quoted in the graph and table represents past performance; past performance is not predictive of future results. The investment return and principal value of shares of the Fund fluctuate, so your shares, when sold, may be worth more or less than their original cost. Current performance of the Fund may be lower or higher than the performance quoted. The performance shown does not reflect the deduction of taxes, if any, that a shareholder would pay on Fund distributions or the redemption of Fund shares. Performance data current to the most recent month end may be obtained by calling 1-888-309-3539.

Investors should read the Fund’s prospectus with regard to the Fund’s investment objectives, risks, and charges and expenses in conjunction with these financial statements. Employee retirement benefit plans that invest plan assets in the Separate Investment Accounts (SIAs) may be subject to certain charges as set forth in their respective Plan Documents. Total return figures would be lower for the periods presented if they reflected these charges.

13

What is the investment approach of MassMutual Core Bond Fund*, and who is the Fund’s subadviser?

The Fund seeks to achieve a high total rate of return consistent with prudent investment risk and the preservation of capital by investing primarily in a diversified portfolio of investment grade fixed income securities. Under normal circumstances, the Fund invests at least 80% of its net assets (plus the amount of any borrowings for investment purposes) in investment grade fixed income securities (rated Baa3 or higher by Moody’s, BBB- or higher by Standard & Poor’s or the equivalent by any nationally recognized statistical rating organization, or, if unrated, determined to be of comparable quality by the subadviser). The Fund’s subadviser is Barings LLC (Barings). Effective May 1, 2021, Baring International Investment Limited was added as sub-subadviser to the Fund.

How did the Fund perform during the 12 months ended September 30, 2021?

The Fund’s Class R5 shares returned 3.19%, outperforming the -0.90% return of the Bloomberg U.S. Aggregate Bond Index (the “benchmark”), which measures the performance of the investment grade, U.S. dollar-denominated, fixed-rate taxable bond market, including Treasuries, government-related and corporate securities, agency mortgage-backed securities (MBS) (agency fixed-rate and hybrid ARM pass-throughs), asset-backed securities (ABS), and commercial mortgage-backed securities (CMBS). (“Agency” refers to debt issued by U.S. federal government agencies or government-sponsored entities for financing purposes.)

For a discussion on the economic and market environment during the 12-month period ended September 30, 2021, please see the Economic and Market Overview, beginning on page 3.

Subadviser discussion of factors that contributed to the Fund’s performance

For the year ended September 30, 2021, ABS was a top contributor to the Fund’s performance. During the period, the Fund held an overweight allocation to the sector, relative to the benchmark. The asset class underwent significant spread tightening over the period as a result of improving consumer fundamentals heading out of the COVID-19 pandemic. (Credit spreads are the differences in yields between comparable bonds, with widening spreads often indicating increasing yields and decreasing prices for bonds.) The consumer and commercial ABS markets had the tailwind of strong and improving fundamentals, giving momentum to tighter spreads across several collateral subsectors. This theme has been consistent for the past several months. CMBS and residential mortgage-backed securities (RMBS) both contributed positively to performance over the period. On the other hand, both the municipal allocation and the AA quality allocation detracted from performance over the period. An underweight allocation to Treasuries contributed positively to performance.

The Fund’s overweight allocation to investment-grade corporates was a positive contributor to performance during the reporting period. Specifically, Fund holdings in the secondary oil & gas producers, gas pipelines, and life insurance industries contributed to returns. Conversely, Fund holdings in the aerospace, electric utilities, and health care facilities industries detracted from performance over the period.

The Fund used derivative instruments for yield curve, duration, downside hedging, and to gain exposures. They may also be used as a substitute as a direct investment. In aggregate, these positions detracted from performance over the reporting period. Derivatives are securities that derive their value from the performance of one or more other investments and take the form of a contract between two or more parties. Derivatives can be used for hedging, speculation, or both.

Subadviser outlook

Market trends finished the period showing strong signs of improvement. Minutes released from the Federal Reserve’s (the Fed) latest meeting confirmed that the Federal Open Market Committee (FOMC) will continue its asset purchases as the path of economic recovery continues to be dictated by the direction and trends of the COVID-19 virus. Nevertheless, Fed Chair Powell reiterated the Fed’s commitment to its dual mandate of promoting maximum employment and fostering price stability, while also noting that economic indicators point to progress on both fronts. As we move forward, Barings believes that the Fed will prudently approach the market concern over inflation, and act appropriately as economic indicators continue to shape the “reopening” story. While there are still unknowns leading into the fourth quarter of 2021, Fund management plans to maintain a defensive stance, while remaining diligent to Barings’ fundamental bottom-up process of capital preservation.

14

15

GROWTH OF $10,000 INVESTMENT - Class R5

The graph above illustrates a representative class of the Fund’s historical performance for the past 10 fiscal years or since inception (for Funds lacking 10-year records) in comparison to its benchmark index, as well as one or more additional indexes, if applicable. The performance of other share classes will be greater than or less than the class depicted above.

Generally accepted accounting principles require adjustments to be made to the net assets of the Fund at period end for financial reporting purposes only, and as such, the total return based on the unadjusted net asset value per share may differ from the total return reported in the financial highlights.

Performance results in the graph and table are shown both with and without the imposition of the maximum applicable sales charge (if applicable) and reflect the percentage change in net asset value, including reinvestment of dividends and capital gains distributions. Investors should note that the Fund is a professionally managed mutual fund, while the index or indexes shown above are unmanaged, cannot be purchased directly, and, with the exception of any peer group index, do not incur expenses.

Performance data quoted in the graph and table represents past performance; past performance is not predictive of future results. The investment return and principal value of shares of the Fund fluctuate, so your shares, when sold, may be worth more or less than their original cost. Current performance of the Fund may be lower or higher than the performance quoted. The performance shown does not reflect the deduction of taxes, if any, that a shareholder would pay on Fund distributions or the redemption of Fund shares. Performance data current to the most recent month end may be obtained by calling 1-888-309-3539.

Investors should read the Fund’s prospectus with regard to the Fund’s investment objectives, risks, and charges and expenses in conjunction with these financial statements. Employee retirement benefit plans that invest plan assets in the Separate Investment Accounts (SIAs) may be subject to certain charges as set forth in their respective Plan Documents. Total return figures would be lower for the periods presented if they reflected these charges.

16

What is the investment approach of MassMutual Diversified Bond Fund*, and who is the Fund’s subadviser?

The Fund seeks a superior total rate of return by investing in fixed income instruments. Under normal circumstances, the Fund invests at least 80% of its net assets (plus the amount of any borrowings for investment purposes) in fixed income securities. The Fund’s subadviser is Barings LLC (Barings). Effective May 1, 2021, Baring International Investment Limited was added as sub-subadviser to the Fund.

How did the Fund perform during the 12 months ended September 30, 2021?

The Fund’s Class R5 shares returned 5.79%, significantly outperforming the -0.90% return of the Bloomberg U.S. Aggregate Bond Index (the “benchmark”), which measures the performance of the investment grade, U.S. dollar-denominated, fixed-rate taxable bond market, including Treasuries, government-related and corporate securities, agency mortgage-backed securities (MBS) (agency fixed-rate and hybrid ARM pass-throughs), asset-backed securities (ABS), and commercial mortgage-backed securities (CMBS). (“Agency” refers to debt issued by U.S. federal government agencies or government-sponsored entities for financing purposes.)

For a discussion on the economic and market environment during the 12-month period ended September 30, 2021, please see the Economic and Market Overview, beginning on page 3.

Subadviser discussion of factors that contributed to the Fund’s performance

For the period ended September 30, 2021, the Fund’s overweight allocation to investment-grade corporate securities was a primary contributor to performance. Industry allocations to secondary oil & gas producers and gas pipelines also contributed to returns, while pharmaceuticals and wireless allocations detracted. Corporate spreads continued to ratchet tighter and finished the period well inside of two-year average levels. (Credit spreads are the differences in yields between comparable bonds, with widening spreads often indicating increasing yields and decreasing prices for bonds.) The Fund’s outperformance within the corporate credit sector was a result of both security selection and its overweight allocation to the sector relative to the benchmark. An overweight allocation to high-yield corporates also contributed to returns.

ABS was also a top contributor to performance during the reporting period, as the Fund held an overweight allocation to the sector. The asset class underwent significant spread tightening over the period as a result of improving consumer fundamentals heading out of the COVID-19 pandemic. The consumer and commercial ABS markets had the tailwind of strong and improving fundamentals giving momentum to tighter spreads across several collateral subsectors. This theme has been consistent for the past several months. CMBS and residential mortgage-backed securities (RMBS) both contributed positively to performance over the period. On the other hand, the municipal allocation detracted from returns.

An allocation to high-quality short-weighted average life collateralized loan obligations (CLOs) contributed to the Fund’s performance; the Fund has approximately a 4-5% allocation to the out-of-benchmark sector. The Fund’s underweight allocation to Treasuries was also additive.

The Fund uses derivative instruments for yield curve, duration, downside hedging, and to gain exposures. They may also be used as a substitute as a direct investment. (Derivatives are securities that derive their value from the performance of one or more other investments and take the form of a contract between two or more parties.) In aggregate, these positions detracted marginally from the Fund’s performance over the year.

Subadviser outlook

Market trends finished the period showing strong signs of improvement. Minutes released from the Federal Reserve’s (the Fed) latest meeting confirmed that the Federal Open Market Committee (FOMC) will continue its asset purchases as the path of economic recovery continues to be dictated by the direction and trends of the COVID-19 virus. Nevertheless, Fed Chair Powell reiterated the Fed’s commitment to its dual mandate of promoting maximum employment and fostering price stability, while also noting that economic indicators point to progress on both fronts. As we move forward, Barings believes that the Fed

17

will prudently approach market concerns over inflation, and act appropriately as economic indicators continue to shape the “reopening” story. While there are still unknowns leading into the fourth quarter of 2021, Fund management plans to maintain a defensive stance, while remaining diligent to Barings’ fundamental bottom-up process of capital preservation.

At period-end, the Fund remained overweight in sectors such as corporates and securitized relative to liquid products, such as governments and agencies.

18

GROWTH OF $10,000 INVESTMENT - Class R5

The graph above illustrates a representative class of the Fund’s historical performance for the past 10 fiscal years or since inception (for Funds lacking 10-year records) in comparison to its benchmark index, as well as one or more additional indexes, if applicable. The performance of other share classes will be greater than or less than the class depicted above.

Generally accepted accounting principles require adjustments to be made to the net assets of the Fund at period end for financial reporting purposes only, and as such, the total return based on the unadjusted net asset value per share may differ from the total return reported in the financial highlights.

Performance results in the graph and table are shown both with and without the imposition of the maximum applicable sales charge (if applicable) and reflect the percentage change in net asset value, including reinvestment of dividends and capital gains distributions. Investors should note that the Fund is a professionally managed mutual fund, while the index or indexes shown above are unmanaged, cannot be purchased directly, and, with the exception of any peer group index, do not incur expenses.

Performance data quoted in the graph and table represents past performance; past performance is not predictive of future results. The investment return and principal value of shares of the Fund fluctuate, so your shares, when sold, may be worth more or less than their original cost. Current performance of the Fund may be lower or higher than the performance quoted. The performance shown does not reflect the deduction of taxes, if any, that a shareholder would pay on Fund distributions or the redemption of Fund shares. Performance data current to the most recent month end may be obtained by calling 1-888-309-3539.

Investors should read the Fund’s prospectus with regard to the Fund’s investment objectives, risks, and charges and expenses in conjunction with these financial statements. Employee retirement benefit plans that invest plan assets in the Separate Investment Accounts (SIAs) may be subject to certain charges as set forth in their respective Plan Documents. Total return figures would be lower for the periods presented if they reflected these charges.

19

What is the investment approach of MassMutual High Yield Fund*, and who is the Fund’s subadviser?

The Fund seeks to achieve a high level of total return, with an emphasis on current income, by investing primarily in high yield debt and related securities. The Fund invests primarily in lower rated U.S. debt securities (“junk” or “high yield” bonds), including securities in default. Under normal circumstances, the Fund invests at least 80% of its net assets (plus the amount of any borrowings for investment purposes) in lower rated fixed income securities (rated below Baa3 by Moody’s, below BBB- by Standard & Poor’s or the equivalent by any nationally recognized statistical rating organization (using the lower rating) or, if unrated, determined to be of below investment grade quality by the Fund’s subadviser). The Fund’s subadviser is Barings LLC (Barings).

How did the Fund perform during the 12 months ended September 30, 2021?

The Fund’s Service shares returned 14.00%, outperforming the 11.28% return of the Bloomberg U.S. Corporate High-Yield Bond Index (the “benchmark”), which measures the performance of U.S. dollar-denominated, non-investment grade, fixed-rate, taxable corporate bonds, including corporate bonds, fixed-rate bullet, puttable, and callable bonds, SEC Rule 144A securities, original issue zeros, pay-in-kind bonds, fixed-rate and fixed-to-floating capital securities.

For a discussion on the economic and market environment during the 12-month period ended September 30, 2021, please see the Economic and Market Overview, beginning on page 3.

Subadviser discussion of factors that contributed to the Fund’s performance

For the one-year period ended September 30, 2021, the U.S. high-yield bond market finished the fiscal year with positive performance, as economies continued to rebound off the 2020 downturn caused by the COVID-19 pandemic and global economic shutdown.

Returns were positive across all industry sectors, with energy leading the way on the back of crude oil prices that nearly doubled from over a year ago. In general, the more cyclical areas of the market drove returns, as the energy and consumer cyclical sectors were the top contributors to return. Lower-quality bonds significantly outperformed their higher quality counterparts, with triple-C and below rated bonds significantly outperforming single-Bs and double-Bs, benefiting from the overall risk-on sentiment experienced during the period.

As it relates to the Fund, both credit selection and an overweight allocation to lower-quality bonds were the key contributors to outperformance relative to the benchmark. The Fund’s positioning within the energy, basic industry, and communications sectors drove the Fund’s outperformance from a credit selection standpoint. The Fund also benefited from overweight positions in pockets of the market that rallied significantly, such as energy and transportation. Across ratings, the Fund benefited from an overweight allocation to triple-Cs and an underweight allocation to double-Bs.

With respect to Fund holdings, the Fund’s top-performing credits were Occidental Petroleum, which explores for, develops, produces, and markets crude oil and natural gas; Avolon, the third-largest aircraft leasing company in the world; and Peabody Energy, a coal mining company. The largest detractors were PBF Holding, an independent American refiner; Jonah Energy, a private exploration & production (E&P) company; and Coinbase Global, an American company that operates a cryptocurrency exchange platform.

Subadviser outlook

Looking ahead to the coming months, and with many investors still facing challenges when it comes to meeting their yield targets, Barings expects to see continued demand for high-yield assets on a strategic basis. In Fund management’s view, much of the recent market concerns around the Delta variant and other factors that are disrupting supply-chain dynamics overlook the fact that many high-yield companies today have strong balance sheets and remain supported by ample liquidity. Healthy debt issuance trends over the past year have enabled companies to refinance their near-term debt obligations and shore up their liquidity profiles. As a result, Barings expects most companies could remain well capitalized to withstand near-term market

20

disruptions, and hence default situations could come down and remain manageable through the end of 2021. Fund management plans to continue to monitor the recent rise in interest rates, which remains an area of focus as the global economy regains strength and inflation becomes a more prevalent theme. However, Barings believes that high yield remains an asset class that is more resilient to higher rates and could benefit greatly from a stronger economy. Furthermore, Barings notes that the Fund also has some exposure to bank loans, which have historically been less sensitive to rising rates due to the floating rate coupon.

21

GROWTH OF $10,000 INVESTMENT - Service Class

The graph above illustrates a representative class of the Fund’s historical performance for the past 10 fiscal years or since inception (for Funds lacking 10-year records) in comparison to its benchmark index, as well as one or more additional indexes, if applicable. The performance of other share classes will be greater than or less than the class depicted above.

Generally accepted accounting principles require adjustments to be made to the net assets of the Fund at period end for financial reporting purposes only, and as such, the total return based on the unadjusted net asset value per share may differ from the total return reported in the financial highlights.

Performance results in the graph and table are shown both with and without the imposition of the maximum applicable sales charge (if applicable) and reflect the percentage change in net asset value, including reinvestment of dividends and capital gains distributions. Investors should note that the Fund is a professionally managed mutual fund, while the index or indexes shown above are unmanaged, cannot be purchased directly, and, with the exception of any peer group index, do not incur expenses.

Performance data quoted in the graph and table represents past performance; past performance is not predictive of future results. The investment return and principal value of shares of the Fund fluctuate, so your shares, when sold, may be worth more or less than their original cost. Current performance of the Fund may be lower or higher than the performance quoted. The performance shown does not reflect the deduction of taxes, if any, that a shareholder would pay on Fund distributions or the redemption of Fund shares. Performance data current to the most recent month end may be obtained by calling 1-888-309-3539.

Investors should read the Fund’s prospectus with regard to the Fund’s investment objectives, risks, and charges and expenses in conjunction with these financial statements. Employee retirement benefit plans that invest plan assets in the Separate Investment Accounts (SIAs) may be subject to certain charges as set forth in their respective Plan Documents. Total return figures would be lower for the periods presented if they reflected these charges.

22

What is the investment approach of MassMutual Balanced Fund*, and who is the Fund’s subadviser?

The Fund seeks a high total return by investing across different asset classes (U.S. equity securities and fixed income securities) each represented by a different segment of the Fund’s portfolio. Under normal market conditions, the Fund’s subadviser and sub-subadviser expect that 55%-75% of the Fund’s net assets will be invested in U.S. equity securities (the “U.S. Equity Segment”) and 25%-45% of the Fund’s net assets will be invested in fixed income securities (the “Bond Segment”) that meet certain environmental, social, and governance (“ESG”) criteria. The Fund will target a long term strategic allocation of 65% to the U.S. Equity Segment and 35% to the Bond Segment. The Fund’s subadviser is Invesco Advisers, Inc., and its sub-subadviser is Invesco Capital Management LLC (together with Invesco Advisers, Inc., “Invesco”). Effective November 18, 2020, Invesco Advisers, Inc. replaced Barings LLC as subadviser of the Fund and Invesco Capital Management LLC was added as sub-subadviser to the Fund.

How did the Fund perform during the 12 months ended September 30, 2021?

The Fund’s Class R5 shares returned 19.58%, significantly underperforming the 30.00% return of the S&P 500^® Index (the “stock component’s benchmark”), which measures the performance of 500 widely held stocks in the U.S. equity market. The Fund’s Class R5 shares significantly outperformed the -0.90% return of the Bloomberg U.S. Aggregate Bond Index (the “bond component’s benchmark”), which measures the performance of the investment grade, U.S. dollar-denominated, fixed-rate taxable bond market, including Treasuries, government-related and corporate securities, mortgage-backed securities (MBS) (agency fixed-rate and hybrid ARM pass-throughs), asset-backed securities (ABS), and commercial mortgage-backed securities (CMBS). (“Agency” refers to debt issued by U.S. federal government agencies or government-sponsored entities for financing purposes.) Additionally, the Fund’s Class R5 shares outperformed the 18.64% return of the Lipper Balanced Fund Index, an unmanaged, equally weighted index of the 30 largest mutual funds within the Lipper Balanced Category; and outperformed the 18.50% return of the Custom Balanced Index, which comprises the stock component’s benchmark and the bond component’s benchmark. The weightings of each index in the Custom Balanced Index are 65% and 35%, respectively.

For a discussion on the economic and market environment during the 12-month period ended September 30, 2021, please see the Economic and Market Overview, beginning on page 3.

Subadviser discussion of factors that contributed to the Fund’s stock component performance

During the portion of the period that Invesco served as subadviser of the Fund (November 18, 2020–September 30, 2021), top detractors from performance in the stock component included the Fund’s exposure to the communication services and financial sectors. Contributing to performance, on the other hand, were the Fund’s allocations to the information technology and consumer discretionary sectors. The Fund’s equity component (what Invesco refers to as the “equity sleeve”) is passively managed and seeks to closely track the investment results, before fees and expenses, of the Invesco US Large Cap Total Balanced Multi-Factor ESG Index (“the Index”). The Index employs a security-ranking model applied to U.S. large-capitalization equities with best-in-class Environmental, Social, and Governance (ESG) scores, while excluding certain industries (e.g., tobacco, aerospace, coal, and pipelines) and controversial companies. The model is based on four well-established factors, which are equally weighted: quality, value, price momentum, and low volatility. (The equity sleeve previously tracked the Invesco US Multi-Factor ESG 150 Index)

While the equity sleeve’s quality factor lagged during the period, the value factor switched from being a performance detractor to a performance leader, relative to the overall market. During the reporting period, the majority of the equity sleeve’s contribution to performance came from the consumer cyclicals sector.

During the portion of the period that Barings served as subadviser of the Fund (October 1–November 17, 2020), Fund holdings General Electric, The Walt Disney Co., and Boeing detracted from performance, whereas the real estate sector contributed to returns. Barings’ allocation to international holdings was a contributor to Fund returns during this period.

23

Subadviser discussion of factors that contributed to the Fund’s bond component performance

During the reporting period of November 18, 2020–September 30, 2021, the Fund’s fixed income component (what Invesco refers to as the “fixed income sleeve”) had a negative total return, as interest rates generally moved higher during the period. The fixed income sleeve is an actively managed, Intermediate Core ESG fixed income strategy that seeks to deliver income, growth potential, and serve as a ballast to other risk assets. The strategy integrates ESG factors into the fundamental evaluation of investment opportunities where such factors are material and relevant to the credit quality of the investment opportunity. Certain industries that are excluded from the strategy include alcohol producers, tobacco producers, gambling operators, nuclear power, conventional weapons manufacturing, cluster bomb manufacturer, and civilian firearms producers.

During the reporting period, underweight allocations to emerging markets and taxable municipal issuers detracted from Invesco’s performance. On the other hand, the most significant contributor to returns was an overweight allocation to investment-grade corporate issuers, specifically in the financial services and technology, media, and telecommunications sectors. Security selection in the financial services and consumer non-cyclical sectors also aided returns. The fixed income component held a shorter-than-benchmark duration profile, which also aided relative returns as interest rates rose during the period. Duration is a measure of a bond fund’s sensitivity to interest rates. The longer the duration, the greater the price impact on the bond or portfolio when interest rates rise or fall. During the reporting period, Invesco used derivatives in the fixed income component for risk management purposes, to reduce duration by approximately half a year. The performance impact of the derivatives was minimal overall. (Derivatives are securities that derive their value from the performance of one or more other investments and take the form of a contract between two or more parties. Derivatives can be used for hedging, speculation, or both.)

While Barings served as subadviser of the Fund, within the fixed income component, corporate industry allocations to aerospace and health care facilities detracted from performance. Conversely, the corporate allocation contributed to returns, with underlying financial, industrial, and utility sector positioning all helping to drive performance. Securitized allocations across commercial and consumer ABS, CMBS, and residential mortgage-backed securities (RMBS) also contributed to the fixed income component’s performance during the period. (Bond issuers create securitized bonds by pooling various types of contractual debt, such as mortgages, auto loans, or credit card debt.) The use of derivatives for downside protection and hedging purposes detracted from performance during the period.

Subadviser outlook

Given Invesco’s expectation of a strong economic recovery, Fund management believes the environment could favor value over growth, and the industrials, materials, and financial sectors over more defensive areas in the short term. In fixed income, Fund management continues to expect high-yield and credit issues to outperform government bonds. Invesco also expects commodity prices could continue to benefit from inflationary pressures in the short term. Invesco also believes that the markets could experience more volatility in the near term, which is why Fund management’s view is that investors could benefit from broad diversification across and within asset classes.

24

25

GROWTH OF $10,000 INVESTMENT - Class R5

The graph above illustrates a representative class of the Fund’s historical performance for the past 10 fiscal years or since inception (for Funds lacking 10-year records) in comparison to its benchmark index, as well as one or more additional indexes, if applicable. The performance of other share classes will be greater than or less than the class depicted above.

Generally accepted accounting principles require adjustments to be made to the net assets of the Fund at period end for financial reporting purposes only, and as such, the total return based on the unadjusted net asset value per share may differ from the total return reported in the financial highlights.

Performance results in the graph and table are shown both with and without the imposition of the maximum applicable sales charge (if applicable) and reflect the percentage change in net asset value, including reinvestment of dividends and capital gains distributions. Investors should note that the Fund is a professionally managed mutual fund, while the index or indexes shown above are unmanaged, cannot be purchased directly, and, with the exception of any peer group index, do not incur expenses.

Performance data quoted in the graph and table represents past performance; past performance is not predictive of future results. The investment return and principal value of shares of the Fund fluctuate, so your shares, when sold, may be worth more or less than their original cost. Current performance of the Fund may be lower or higher than the performance quoted. The performance shown does not reflect the deduction of taxes, if any, that a shareholder would pay on Fund distributions or the redemption of Fund shares. Performance data current to the most recent month end may be obtained by calling 1-888-309-3539.

Investors should read the Fund’s prospectus with regard to the Fund’s investment objectives, risks, and charges and expenses in conjunction with these financial statements. Employee retirement benefit plans that invest plan assets in the Separate Investment Accounts (SIAs) may be subject to certain charges as set forth in their respective Plan Documents. Total return figures would be lower for the periods presented if they reflected these charges.

26

What is the investment approach of MassMutual Disciplined Value Fund*, and who is the Fund’s subadviser?

The Fund seeks long-term total return by investing primarily in equity securities of U.S. large- and mid-cap companies. The Fund currently invests substantially all of its assets in companies represented in the MSCI USA Value Index or the Russell 1000^® Value Index at the time of purchase, although the Fund is actively managed and is not an index fund or a passively managed investment. Constituents of the MSCI USA Value Index and the Russell 1000 Value Index are companies that exhibit certain value characteristics, as defined by the index providers, such as lower price-to-book ratios, lower prices relative to forecasted earnings, and higher dividend yields. The Fund’s subadviser is Wellington Management Company LLP (Wellington Management). Effective November 18, 2020, Wellington Management replaced Barings LLC as subadviser of the Fund.

How did the Fund perform during the 12 months ended September 30, 2021?

The Fund’s Service Class shares returned 37.60%, outperforming the 35.01% return of the Russell 1000 Value Index (the “benchmark”), which measures the performance of the large-cap value segment of U.S. equity securities. It includes the Russell 1000 Index companies with lower price-to-book ratios and lower expected growth values. It is market-capitalization weighted.

For a discussion on the economic and market environment during the 12-month period ended September 30, 2021, please see the Economic and Market Overview, beginning on page 3.

Subadviser discussion of factors that contributed to the Fund’s performance

During the portion of the period that Wellington Management served as a subadviser of the Fund (November 18, 2020–September 30, 2021), the impact of security selection, which is a byproduct of Wellington Management’s systematic factor construction process, proved positive, led by strong stock selection in the health care, communication services, and energy sectors. This was only partially offset by weaker stock selection in the financial, information technology, and consumer discretionary sectors. Devon Energy (energy), an independent oil and natural gas exploration and production company, and entertainment company Discovery (communication services) were the Fund holdings that were the two largest contributors to relative results during the period. The Fund continued to own Devon Energy as of the end of the period but eliminated its position in Discovery. These positives were partially offset by the Fund’s overweight allocations to weaker-performing stocks, including Bristol-Myers Squibb (health care) and payment services provider Fidelity National Information Services (information technology), which were the largest relative detractors during the period. The Fund reduced its overweight allocation to each of these holdings.

The Fund’s sector allocation contributed to performance during the period, led by underweight allocations to the utilities and communication services sectors, and an overweight allocation to the financial sector. This was partially offset by overweight allocations to the consumer staples and health care sectors, and an underweight allocation to the energy sector, which detracted from relative results.

During the portion of the period that Barings LLC served as a subadviser of the Fund (October 1–November 17, 2020), the Fund lagged the benchmark. The sectors that hampered performance were the industrial, information technology, and energy sectors. With respect to specific Fund holdings, Alphabet (the parent company of Google), Honeywell International Inc. (an American multinational conglomerate corporation), and investment banking company JPMorgan Chase were the strongest performers, while General Electric, Boeing, and The Walt Disney Company detracted.

Subadviser outlook

Wellington Management notes that, while the rollout of vaccines in the U.S. has benefited equity markets, investor optimism has been muddled by increasing macro uncertainties. Within the U.S. value universe, which tends to have greater exposure to macroeconomic risks, Fund management continues to monitor the evolving risks in the universe. In particular, Wellington Management is paying close attention to the potential economic ramifications from the spread of the COVID-19 Delta variant and the reemergence of pandemic-related restrictions in some regions. Wellington Management is also monitoring the potential broad market disruption from political uncertainty surrounding the U.S. debt ceiling, and the potential impact that Federal Reserve tapering may have on company fundamentals. Against this backdrop, Wellington Management expects continued volatility, as investors balance near-term risks with long-term opportunities.

27

28

GROWTH OF $10,000 INVESTMENT - Service Class

The graph above illustrates a representative class of the Fund’s historical performance for the past 10 fiscal years or since inception (for Funds lacking 10-year records) in comparison to its benchmark index, as well as one or more additional indexes, if applicable. The performance of other share classes will be greater than or less than the class depicted above.

Generally accepted accounting principles require adjustments to be made to the net assets of the Fund at period end for financial reporting purposes only, and as such, the total return based on the unadjusted net asset value per share may differ from the total return reported in the financial highlights.

Performance results in the graph and table are shown both with and without the imposition of the maximum applicable sales charge (if applicable) and reflect the percentage change in net asset value, including reinvestment of dividends and capital gains distributions. Investors should note that the Fund is a professionally managed mutual fund, while the index or indexes shown above are unmanaged, cannot be purchased directly, and, with the exception of any peer group index, do not incur expenses.

Performance data quoted in the graph and table represents past performance; past performance is not predictive of future results. The investment return and principal value of shares of the Fund fluctuate, so your shares, when sold, may be worth more or less than their original cost. Current performance of the Fund may be lower or higher than the performance quoted. The performance shown does not reflect the deduction of taxes, if any, that a shareholder would pay on Fund distributions or the redemption of Fund shares. Performance data current to the most recent month end may be obtained by calling 1-888-309-3539.

Investors should read the Fund’s prospectus with regard to the Fund’s investment objectives, risks, and charges and expenses in conjunction with these financial statements. Employee retirement benefit plans that invest plan assets in the Separate Investment Accounts (SIAs) may be subject to certain charges as set forth in their respective Plan Documents. Total return figures would be lower for the periods presented if they reflected these charges.

29

What is the investment approach of MassMutual Main Street Fund*, and who is the Fund’s subadviser?

The Fund seeks a high total return by investing primarily in common stocks of U.S. companies of different capitalization ranges. The Fund’s subadviser currently focuses on “large capitalization” issuers, which are considered to be companies with market capitalizations at the time of purchase within the market capitalization range of companies included within the Russell 1000^® Index, although it may purchase stocks of companies with any market capitalization. The Fund’s subadviser is Invesco Advisers, Inc. (Invesco).

How did the Fund perform during the 12 months ended September 30, 2021?

The Fund’s Class R5 shares returned 28.11%, underperforming the 30.00% return of the S&P 500^® Index (the “benchmark”), which measures the performance of 500 widely held stocks in the U.S. equity market.

For a discussion on the economic and market environment during the 12-month period ended September 30, 2021, please see the Economic and Market Overview, beginning on page 3.

Subadviser discussion of factors that contributed to the Fund’s performance

For the period ended September 30, 2021, stock selection in the industrials, communication services, and energy sectors was the primary driver of the Fund’s underperformance. This was partially offset by stronger stock selection in the information technology, health care, and utilities sectors.

For the Fund, the largest individual detractor from relative performance was not owning Alphabet, which is Google’s parent company. With respect to Fund holdings, the largest relative detractors included Amazon, Procter & Gamble, and Verizon. Alphabet was one of the top performers in the benchmark, although it was not in the Fund’s portfolio, as Fund management preferred Amazon, Facebook, and Microsoft for similar exposures. Amazon and Procter & Gamble were both up modestly on an absolute basis but trailed the broader market after previously being outperformers. The companies had performed well as “pandemic plays,” but reopening-oriented stocks began to do better during the trailing one-year period. Verizon is traditionally a more defensive-oriented stock and trailed during the market advance. Additionally, the company continued to experience weak net subscriber additions as COVID-19-related retail store restrictions limited the pool of switching subscribers and AT&T used aggressive pricing to defend its subscriber base.

The Fund holdings that were the top individual contributors to relative returns were Applied Materials, Capital One, and HCA Healthcare. Semiconductor chip manufacturer Applied Materials continued to benefit from a strong fundamental backdrop, as secular demand drivers remained intact and the company successfully navigated supply chain disruptions. Bank holding company Capital One has been a strong performer as the U.S. has continued its path to reopening. HCA Healthcare benefited from strong admissions, with surgeries returning to pre-pandemic levels.

Subadviser outlook

Invesco notes that the rebound in the economy has been substantial. Beyond the pent-up demand caused by cabin fever, this was not a recession caused by excesses that needed to be burned off with time. That said, Fund management does not think a simple resumption of the previous economic trend is in the cards. In Invesco’s view, some areas of the economy will be far slower to recover from the pandemic than others. Fund management believes that some of the business adaptations from COVID-19 are likely to remain as permanent changes, which could reduce demand for commercial travel and real estate.

Further, Invesco notes that copious monetary and fiscal stimulus is adding to an already significant money supply and federal budget deficit, raising the risk of material inflation. Interest rates remain low for now, but Invesco thinks long-term rates bear watching. Fund management believes that the current debate in the market is whether or not the elevated inflation we have been experiencing of late is transitory.

30

31

GROWTH OF $10,000 INVESTMENT - Class R5

The graph above illustrates a representative class of the Fund’s historical performance for the past 10 fiscal years or since inception (for Funds lacking 10-year records) in comparison to its benchmark index, as well as one or more additional indexes, if applicable. The performance of other share classes will be greater than or less than the class depicted above.

Generally accepted accounting principles require adjustments to be made to the net assets of the Fund at period end for financial reporting purposes only, and as such, the total return based on the unadjusted net asset value per share may differ from the total return reported in the financial highlights.

Performance results in the graph and table are shown both with and without the imposition of the maximum applicable sales charge (if applicable) and reflect the percentage change in net asset value, including reinvestment of dividends and capital gains distributions. Investors should note that the Fund is a professionally managed mutual fund, while the index or indexes shown above are unmanaged, cannot be purchased directly, and, with the exception of any peer group index, do not incur expenses.

Performance data quoted in the graph and table represents past performance; past performance is not predictive of future results. The investment return and principal value of shares of the Fund fluctuate, so your shares, when sold, may be worth more or less than their original cost. Current performance of the Fund may be lower or higher than the performance quoted. The performance shown does not reflect the deduction of taxes, if any, that a shareholder would pay on Fund distributions or the redemption of Fund shares. Performance data current to the most recent month end may be obtained by calling 1-888-309-3539.

Investors should read the Fund’s prospectus with regard to the Fund’s investment objectives, risks, and charges and expenses in conjunction with these financial statements. Employee retirement benefit plans that invest plan assets in the Separate Investment Accounts (SIAs) may be subject to certain charges as set forth in their respective Plan Documents. Total return figures would be lower for the periods presented if they reflected these charges.

32

What is the investment approach of MassMutual Disciplined Growth Fund*, and who is the Fund’s subadviser?

The Fund seeks long-term total return by investing primarily in equity securities of U.S. large- and mid-cap companies. The Fund currently invests substantially all of its assets in companies represented in the MSCI USA Growth Index or the Russell 1000^® Growth Index at the time of purchase, although the Fund is actively managed and is not an index fund or a passively managed investment. Constituents of the MSCI USA Growth Index and the Russell 1000 Growth Index are companies that exhibit certain growth characteristics, as defined by the index providers, such as historical and predicted future earnings per share growth rates and trends, historical sales per share growth trends, and internal growth rate. The Fund’s subadviser is Wellington Management Company LLP (Wellington Management). Effective November 18, 2020, Wellington Management replaced Barings LLC as subadviser of the Fund.

How did the Fund perform during the 12 months ended September 30, 2021?

The Fund’s Service Class shares returned 26.53%, underperforming the 27.32% return of the Russell 1000 Growth Index (the “benchmark”), which measures the performance of the large-cap growth segment of U.S. equity securities. It includes the Russell 1000 Index companies with higher price-to-book ratios and higher forecasted growth values. It is market capitalization weighted.

For a discussion on the economic and market environment during the 12-month period ended September 30, 2021, please see the Economic and Market Overview, beginning on page 3.

Subadviser discussion of factors that contributed to the Fund’s performance

During the portion of the period that Wellington Management served as a subadviser of the Fund (November 18, 2020–September 30, 2021), underweight allocations to the consumer discretionary and health care sectors detracted from relative results. This was partially offset by an underweight allocation to the industrial sector and overweight allocations to the financial and information technology sectors, both of which contributed to performance.

During the period, overweight allocations to Fund holdings Zoom Video Communications (information technology) and Mastercard (information technology) hampered the Fund’s performance. The Fund continued to own both Zoom Video Communications and Mastercard as of September 30, 2021 but reduced its overweight allocation to both names.

On the positive side, the impact of security selection, which is a byproduct of Wellington Management’s systematic factor construction process, was positive for the period, led by strong selection in the industrial, consumer discretionary, and real estate sectors. With respect to specific Fund holdings, technology research and consulting company Gartner (information technology) and enterprise security company Fortinet (information technology) were the two largest contributors to relative results during the period. The Fund continued to own Gartner as of the end of the period but eliminated its position in Fortinet.

During the portion of the period that Barings LLC served as a subadviser of the Fund (October 1-November 17, 2020), the Fund lagged the benchmark. The sectors that hampered performance were consumer discretionary, health care, and consumer staples. With respect to specific Fund holdings, semiconductors and software manufacturer Qualcomm, Zoom Video Communications Inc, and biopharmaceutical company AbbVie were the strongest performers, while positions in Tesla, Mastercard, and NVIDIA, which designs and manufactures computer graphics processors, chipsets, and related multimedia software, detracted from results.

Subadviser outlook

Wellington Management notes that, while the rollout of vaccines in the U.S. has benefited equity markets, investor optimism has been muddled by increasing macro uncertainties. Although U.S. growth stocks ended the period higher, Fund management continues to monitor the evolving risks in the universe. In particular, Wellington Management is paying close attention to the potential economic ramifications from the spread of the COVID-19 Delta variant and the reemergence of pandemic-related restrictions in some regions. Wellington Management is also monitoring the potential broad market disruption from political uncertainty surrounding the U.S. debt ceiling, and the potential impact that Federal Reserve tapering may have on company fundamentals. Against this backdrop, Wellington Management expects continued volatility, as investors balance near-term risks with long-term opportunities.

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34

GROWTH OF $10,000 INVESTMENT - Service Class

The graph above illustrates a representative class of the Fund’s historical performance for the past 10 fiscal years or since inception (for Funds lacking 10-year records) in comparison to its benchmark index, as well as one or more additional indexes, if applicable. The performance of other share classes will be greater than or less than the class depicted above.

Generally accepted accounting principles require adjustments to be made to the net assets of the Fund at period end for financial reporting purposes only, and as such, the total return based on the unadjusted net asset value per share may differ from the total return reported in the financial highlights.

Performance results in the graph and table are shown both with and without the imposition of the maximum applicable sales charge (if applicable) and reflect the percentage change in net asset value, including reinvestment of dividends and capital gains distributions. Investors should note that the Fund is a professionally managed mutual fund, while the index or indexes shown above are unmanaged, cannot be purchased directly, and, with the exception of any peer group index, do not incur expenses.

Performance data quoted in the graph and table represents past performance; past performance is not predictive of future results. The investment return and principal value of shares of the Fund fluctuate, so your shares, when sold, may be worth more or less than their original cost. Current performance of the Fund may be lower or higher than the performance quoted. The performance shown does not reflect the deduction of taxes, if any, that a shareholder would pay on Fund distributions or the redemption of Fund shares. Performance data current to the most recent month end may be obtained by calling 1-888-309-3539.

Investors should read the Fund’s prospectus with regard to the Fund’s investment objectives, risks, and charges and expenses in conjunction with these financial statements. Employee retirement benefit plans that invest plan assets in the Separate Investment Accounts (SIAs) may be subject to certain charges as set forth in their respective Plan Documents. Total return figures would be lower for the periods presented if they reflected these charges.

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What is the investment approach of MassMutual Small Cap Opportunities Fund*, and who is the Fund’s subadviser?

The Fund seeks capital appreciation by investing primarily in common stocks of small-capitalization U.S. companies that the Fund’s subadviser believes have favorable business trends or prospects based on fundamental analysis. Under normal circumstances, the Fund invests at least 80% of its net assets (plus the amount of any borrowings for investment purposes) in securities of small-cap companies. The Fund’s subadviser is Invesco Advisers, Inc. (Invesco).

How did the Fund perform during the 12 months ended September 30, 2021?

The Fund’s Class A shares returned 48.15%, outperforming the 47.68% return of the Russell 2000^® Index (the “benchmark”), which measures the performance of the small-cap segment of the U.S. equity universe. It is a subset of the Russell 3000 Index and includes approximately 2000 of the smallest securities based on a combination of their market cap and current index membership.

For a discussion on the economic and market environment during the 12-month period ended September 30, 2021, please see the Economic and Market Overview, beginning on page 3.

Subadviser discussion of factors that contributed to the Fund’s performance

For the period ended September 30, 2021, strong stock selection in the information technology, financial, and consumer discretionary sectors was the primary driver of the Fund’s outperformance. This was partially offset by weaker stock selection in the materials sector – and both stock selection and an underweight allocation to the energy sector.

The Fund holdings that were the top contributors to relative performance were Atkore, Korn Ferry, and AutoNation. Atkore, a manufacturer and distributor of electrical, mechanical, and safety infrastructure solutions to power and protect the world, benefited from strong price realization as polyvinyl chloride (PVC) conduit industry-wide was in tight supply. This drove the company’s earnings before interest, tax, depreciation, and amortization (EBITDA) margins to a historically strong level. Korn Ferry, a management consulting firm, experienced a strong rebound in revenues, new business bookings, and earnings. American automotive retailer AutoNation benefited as new vehicle shortages led to high per-vehicle pricing in both new and used cars. The company also announced a significant new share repurchase authorization.

On the downside, Fund holdings that were the top individual detractors from relative performance included Emergent BioSolutions, LHC Group, and Tandem Diabetes Care. Global life sciences company Emergent BioSolutions experienced a contamination event at one of their manufacturing facilities that was producing Johnson & Johnson’s COVID-19 vaccine, causing 15 million vials to be discarded. This issue was widely publicized in the media and impacted the company’s reputation as a contract manufacturer, which was their main growth driver. Consequently, Fund management exited the position. LHC Group, a national provider of health-care services, sold off in the last two months of the reporting period after its closest peer reported a difficult quarter; LHC shares traded down in sympathy. Tandem Diabetes Care sold off early in the fiscal year after reporting modest guidance in an environment of elevated investor expectations. Management also pushed out the timing for the next pipeline product, which was not received well by investors.

Subadviser outlook

Invesco notes that the rebound in the economy has been substantial. Beyond the pent-up demand caused by cabin fever, this was not a recession caused by excesses that needed to be burned off with time. That said, Fund management does not think a simple resumption of the previous economic trend is in the cards. In Invesco’s view, some areas of the economy will be far slower to recover from the pandemic than others. Fund management believes that some of the business adaptations from COVID-19 are likely to remain as permanent changes, which could reduce demand for commercial travel and real estate.

Further, Invesco notes that copious monetary and fiscal stimulus is adding to an already significant money supply and federal budget deficit, raising the risk of material inflation. Interest rates remain low for now, but Invesco thinks long-term rates bear watching. Fund management believes that the current debate in the market is whether or not the elevated inflation we have been experiencing of late is transitory.

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37

GROWTH OF $10,000 INVESTMENT - Class A

The graph above illustrates a representative class of the Fund’s historical performance for the past 10 fiscal years or since inception (for Funds lacking 10-year records) in comparison to its benchmark index, as well as one or more additional indexes, if applicable. The performance of other share classes will be greater than or less than the class depicted above.

Generally accepted accounting principles require adjustments to be made to the net assets of the Fund at period end for financial reporting purposes only, and as such, the total return based on the unadjusted net asset value per share may differ from the total return reported in the financial highlights.

Performance results in the graph and table are shown both with and without the imposition of the maximum applicable sales charge (if applicable) and reflect the percentage change in net asset value, including reinvestment of dividends and capital gains distributions. Investors should note that the Fund is a professionally managed mutual fund, while the index or indexes shown above are unmanaged, cannot be purchased directly, and, with the exception of any peer group index, do not incur expenses.

Performance data quoted in the graph and table represents past performance; past performance is not predictive of future results. The investment return and principal value of shares of the Fund fluctuate, so your shares, when sold, may be worth more or less than their original cost. Current performance of the Fund may be lower or higher than the performance quoted. The performance shown does not reflect the deduction of taxes, if any, that a shareholder would pay on Fund distributions or the redemption of Fund shares. Performance data current to the most recent month end may be obtained by calling 1-888-309-3539.

Investors should read the Fund’s prospectus with regard to the Fund’s investment objectives, risks, and charges and expenses in conjunction with these financial statements. Employee retirement benefit plans that invest plan assets in the Separate Investment Accounts (SIAs) may be subject to certain charges as set forth in their respective Plan Documents. Total return figures would be lower for the periods presented if they reflected these charges.

38

What is the investment approach of MassMutual Global Fund*, and who is the Fund’s subadviser?

The Fund seeks long-term capital appreciation by investing primarily in common stocks of companies in the U.S. and foreign countries. The Fund can invest without limit in foreign securities, including American Depositary Receipts (ADRs), and can invest in any country, including developing or emerging market countries. However, the Fund currently emphasizes investments in developed markets such as the United States, Western European countries, and Japan. As a fundamental policy, the Fund normally will invest in at least three countries (one of which may be the United States). Typically, the Fund invests in a number of different countries. The Fund’s subadviser is Invesco Advisers, Inc. (Invesco).

How did the Fund perform during the 12 months ended September 30, 2021?

The Fund’s Class R5 shares returned 31.27%, outperforming the 27.44% return of the MSCI All Country World Index (ACWI) (the “benchmark”), which measures the performance of the large- and mid-cap segments of all country markets. It is free float-adjusted market-capitalization weighted.

For a discussion on the economic and market environment during the 12-month period ended September 30, 2021, please see the Economic and Market Overview, beginning on page 3.

Subadviser discussion of factors that contributed to the Fund’s performance

For the period ended September 30, 2021, favorable stock selection was the primary factor in the Fund’s outperformance. The Fund benefited most from its holdings in the communication services, real estate, and consumer staples sectors, as well as the industrial, health care, and utilities sectors. Fund holdings in the financial, information technology, and energy sectors underperformed. Over the reporting period the Fund had its largest overweight allocations to the communication services, information technology, and industrial sectors, and its largest underweight allocation to the financial sector.

The three Fund holdings that were major positive contributors to performance for the reporting period were Alphabet Inc., LVMH Moet Hennessy Louis Vuitton, and Intuit Inc. Alphabet Inc., the parent company of Google, has long been the Fund’s largest holding. The company has a collection of businesses that possess large competitive moats. LVMH Moet Hennessy Luis Vuitton, like most luxury goods companies, did surprisingly well through the pandemic. Intuit Inc. is the leading provider of tax and bookkeeping software in the U.S. In Fund management’s view, the company has done an effective job at innovating, and its purchase of Credit Karma is now bearing fruit.

The three Fund holdings that were major detractors from performance during the reporting period were SAP SE; Sarepta Therapeutics, Inc.; and Fidelity National Information Services, Inc. SAP SE, the dominant enterprise software company, is transitioning its clients to a subscription model and this is compressing earnings. Sarepta Therapeutics Inc. is a biotech that had a late-stage clinical trial failure related to a muscular dystrophy therapy. Fund management exited the Fund’s position in Sarepta Therapeutics upon the news. Fidelity National Information Services is a company with a significant, hard-to-dislodge position in the plumbing of the financial system. It has not been the center of investors’ attentions during the strong market uptick.

Subadviser outlook

Over the past 12 months, global equities performed well. Most economies have begun to recover from the COVID-19 recession, while fiscal and monetary authorities have remained accommodative. The reopening of the economy has led to shortages in supply chains as well as inflationary pressures. In Invesco’s opinion, this is to be expected. However, Fund management believes that near-term investor sentiment is likely to be driven by expectations regarding inflation and how soon central banks, especially the Federal Reserve in the U.S., begin to tighten monetary conditions in response.

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40

GROWTH OF $10,000 INVESTMENT - Class R5

The graph above illustrates a representative class of the Fund’s historical performance for the past 10 fiscal years or since inception (for Funds lacking 10-year records) in comparison to its benchmark index, as well as one or more additional indexes, if applicable. The performance of other share classes will be greater than or less than the class depicted above.

Generally accepted accounting principles require adjustments to be made to the net assets of the Fund at period end for financial reporting purposes only, and as such, the total return based on the unadjusted net asset value per share may differ from the total return reported in the financial highlights.

Performance results in the graph and table are shown both with and without the imposition of the maximum applicable sales charge (if applicable) and reflect the percentage change in net asset value, including reinvestment of dividends and capital gains distributions. Investors should note that the Fund is a professionally managed mutual fund, while the index or indexes shown above are unmanaged, cannot be purchased directly, and, with the exception of any peer group index, do not incur expenses.

Performance data quoted in the graph and table represents past performance; past performance is not predictive of future results. The investment return and principal value of shares of the Fund fluctuate, so your shares, when sold, may be worth more or less than their original cost. Current performance of the Fund may be lower or higher than the performance quoted. The performance shown does not reflect the deduction of taxes, if any, that a shareholder would pay on Fund distributions or the redemption of Fund shares. Performance data current to the most recent month end may be obtained by calling 1-888-309-3539.

Investors should read the Fund’s prospectus with regard to the Fund’s investment objectives, risks, and charges and expenses in conjunction with these financial statements. Employee retirement benefit plans that invest plan assets in the Separate Investment Accounts (SIAs) may be subject to certain charges as set forth in their respective Plan Documents. Total return figures would be lower for the periods presented if they reflected these charges.

41

What is the investment approach of MassMutual International Equity Fund*, and who are the Fund’s subadvisers?

The Fund seeks to achieve long-term capital appreciation by investing primarily in common stock of foreign companies. The Fund invests primarily in the common stock of companies that are domiciled or that have their primary operations outside of the United States. Under normal circumstances, the Fund invests at least 80% of its net assets (plus the amount of any borrowings for investment purposes) in securities of foreign companies. The Fund may invest up to 100% of its total assets in such securities. The Fund may invest in emerging markets as well as in developed markets throughout the world. From time to time, the Fund may place greater emphasis on investing in one or more particular regions (such as Asia, Europe, or Latin America). The Fund’s subadvisers are Wellington Management Company LLP (Wellington Management), which managed approximately 74% of the Fund’s portfolio; and Thompson, Siegel & Walmsley LLC (TSW), which was responsible for approximately 26% of the Fund’s portfolio, as of September 30, 2021.

How did the Fund perform during the 12 months ended September 30, 2021?

The Fund’s Class R5 shares returned 17.79%, significantly underperforming the 23.92% return of the MSCI All Country World Index (ACWI) ex USA (the “benchmark”), which measures the performance of the large- and mid-cap segments of the particular regions, excluding U.S. equity securities, including developed and emerging markets. It is free float-adjusted market-capitalization weighted.

For a discussion on the economic and market environment during the 12-month period ended September 30, 2021, please see the Economic and Market Overview, beginning on page 3.

Subadviser discussion of factors that contributed to the Fund’s performance

For the period ended September 30, 2021, within the TSW component of the Fund, holdings in communication services detracted from relative returns. Ubisoft Entertainment SA and Square Enix Holdings Co., Ltd. were the Fund component holdings that were the primary laggards, despite being COVID-19 beneficiaries with improving fundamentals and strong growth outlooks. French videogame creator Ubisoft was another Fund component holding that underperformed after turning in mixed results throughout the period. Square Enix reported a series of strong results and raised expectations for future quarters. This Fund component holding’s mobile game segment performed well, but lackluster sales of a highly anticipated console title, Marvel Avengers, detracted. Fund component holdings in the energy sector also hampered relative performance, with ENEOS Holdings being a primary underperformer. The Japanese oil refiner lagged despite reporting solid results and raising guidance. TSW eliminated its position in the company. Turning to the positive, stock selection in the industrial and materials sectors was a primary driver of TSW’s relative returns. Ashtead Group PLC and Hitachi, Ltd. were among the top performers in the industrial sector. U.K. rental equipment company Ashtead Group performed well after releasing a series of satisfactory results. Japanese industrial equipment manufacturer Hitachi performed well after releasing strong results and guidance above expectations. Fund component holdings ArcelorMittal and Glencore PLC performed well in materials. Global steel manufacturer ArcelorMittal reported strong results, underpinned by sequential volume improvements. Global commodity trading and mining company Glencore reported strong results, primarily driven by higher coal prices.

Within the Wellington Management Fund component, top detractors from relative returns during the period included Reckitt Benckiser (consumer staples) and KAO (consumer staples). The share price of Reckitt Benckiser, a U.K.-based global consumer goods and health care company, fell over the period, as management reported weak results and lowered full-year margin guidance. Wellington Management trimmed the position for opportunities where Fund management sees more upside potential. In its second-quarter earnings release, Japanese chemical and cosmetics company KAO guided for softer 2021 earnings, as they continued to suffer from weaker inbound sales amid the pandemic. Turning to the positive, Fund component holdings that were top contributors to relative performance included Bank of Nova Scotia (financials) and Capgemini (information technology). Shares of Bank of Nova Scotia rose over the period on the back of strong earnings, primarily due to reduced provisions for credit loss. Shares of French IT services and consulting company Capgemini increased over the period. While second-quarter earnings missed consensus estimates, semiannual revenue beat expectations, rising nearly 15% year-over-year.

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Subadviser outlook

TSW notes that corporate profit and cash flow growth have been outstanding over the past year, although input price pressures and tight supply chains could squeeze margins in the near term. In TSW’s view, these problems could begin to ease over the next 12 months as markets adjust. TSW believes that interest rate movements will be a key variable heading into 2022. Despite these concerns, TSW believes that conditions remain favorable for non-U.S. equities –and that an expanding global economy, coupled with improved consumer and business confidence, could keep demand for consumer and capital goods at healthy levels, while tolerance for a somewhat higher inflation rate may provide cover for prices to match input costs.

Wellington Management notes that the market and the economy are shifting. Inflation fears are rising for the first time in decades, which is causing investors to consider whether the era of “free money” is coming to an end. The implications of this shift would be significant. Nevertheless, Wellington Management expects that its Fund component has the potential to be resilient in this environment – noting that its holdings generally have strong pricing power to help them manage inflation pressures, and tend to generate substantial cash flow, which allows them to fund their growth without the need for the capital markets. At the end of the period, the Wellington Management Fund component had its largest overweight allocations to the consumer staples and health care sectors – and its most underweight allocations to the information technology and financial sectors.

43

GROWTH OF $10,000 INVESTMENT - Class R5

The graph above illustrates a representative class of the Fund’s historical performance for the past 10 fiscal years or since inception (for Funds lacking 10-year records) in comparison to its benchmark index, as well as one or more additional indexes, if applicable. The performance of other share classes will be greater than or less than the class depicted above.

Generally accepted accounting principles require adjustments to be made to the net assets of the Fund at period end for financial reporting purposes only, and as such, the total return based on the unadjusted net asset value per share may differ from the total return reported in the financial highlights.

Performance results in the graph and table are shown both with and without the imposition of the maximum applicable sales charge (if applicable) and reflect the percentage change in net asset value, including reinvestment of dividends and capital gains distributions. Investors should note that the Fund is a professionally managed mutual fund, while the index or indexes shown above are unmanaged, cannot be purchased directly, and, with the exception of any peer group index, do not incur expenses.

Performance data quoted in the graph and table represents past performance; past performance is not predictive of future results. The investment return and principal value of shares of the Fund fluctuate, so your shares, when sold, may be worth more or less than their original cost. Current performance of the Fund may be lower or higher than the performance quoted. The performance shown does not reflect the deduction of taxes, if any, that a shareholder would pay on Fund distributions or the redemption of Fund shares. Performance data current to the most recent month end may be obtained by calling 1-888-309-3539.

Investors should read the Fund’s prospectus with regard to the Fund’s investment objectives, risks, and charges and expenses in conjunction with these financial statements. Employee retirement benefit plans that invest plan assets in the Separate Investment Accounts (SIAs) may be subject to certain charges as set forth in their respective Plan Documents. Total return figures would be lower for the periods presented if they reflected these charges.

44

What is the investment approach of MassMutual Strategic Emerging Markets Fund*, and who is the Fund’s subadviser?

The Fund seeks long-term capital growth by investing mainly in common stocks of issuers in developing and emerging markets throughout the world and at times may invest up to 100% of its total assets in foreign securities. Under normal market conditions, the Fund will invest at least 80% of its net assets (plus the amount of any borrowings for investment purposes) in equity securities of issuers whose principal activities are in a developing (or emerging) market, i.e., are in a developing market or are economically tied to a developing market country. The Fund will invest in at least three developing markets. In general, countries may be considered developing or emerging markets if they are included in any one of the MSCI emerging markets indexes, classified as a developing or emerging market, or classified under a similar or corresponding classification, by organizations such as the World Bank and the International Monetary Fund, or have economies, industries, and stock markets with similar characteristics. The Fund’s subadviser is Invesco Advisers, Inc. (Invesco).

How did the Fund perform during the 12 months ended September 30, 2021?

The Fund’s Class R5 shares returned 14.38%, underperforming the 18.20% return of the MSCI Emerging Markets (EM) Index (the “benchmark”), which measures the performance of the large- and mid-cap segments of emerging market equity securities. It is free float-adjusted market-capitalization weighted.

For a discussion on the economic and market environment during the 12-month period ended September 30, 2021, please see the Economic and Market Overview, beginning on page 3.

Subadviser discussion of factors that contributed to the Fund’s performance

On a sector basis, an underweight allocation to the information technology sector was the largest detractor from the Fund’s relative performance for the period ended September 30, 2021. Fund holdings in the communication services sector and stock selection within and an underweight allocation to the materials sector also detracted. On the other hand, Fund holdings in the energy, consumer staples, and real estate sectors were among the top contributors to relative performance. With regard to country exposure, Fund holdings in China and Brazil, as well as an underweight allocation to South Korea, detracted from performance. Conversely, stock selection and an overweight allocation to India, along with overweight allocations to Mexico and Russia, were the main contributors to the Fund’s relative performance.

Fund holdings that were detractors from absolute performance included Alibaba (consumer discretionary), New Oriental Education (consumer discretionary), and TAL Education (consumer discretionary). Alibaba is one of China’s most dominant internet companies and holds the leading position in the sizable e-commerce market. However, the company faced several challenges during the reporting period, including increased competition and a host of regulatory interventions. Fund management had been reducing the Fund’s exposure to Alibaba for several quarters, but fully exited the stock at the beginning of the third quarter of 2021. New Oriental Education (EDU) operates in China’s vast after-school tutoring (AST) market and is the nation’s largest private tutoring provider. The AST market in China experienced volatility due to contemplated reforms the government was looking to implement.

TAL Education also operates in China’s AST market. During the reporting period, the price of Tal shares came under extreme pressure for the same reasons as EDU shares did.

Turning to the positive, Fund holdings that contributed to the Fund’s absolute performance included Taiwan Semiconductor Manufacturing (information technology), Novatek (energy), and Housing Development Financial Corp. (financials). Taiwan Semiconductor Manufacturing, the world’s preeminent semiconductor foundry, benefited from the COVID-19-created stay-at-home era, which spurred consumer spending on modern household appliances and other products that rely on customized smart chips. Novatek, a Russian energy company that has always stood at the forefront of technological and business innovation, found creative ways to add value to its constantly expanding natural gas and condensate resource base. Housing Development Finance Corporation, a long-term holding of the Fund, was the first specialized mortgage company in India. During the period, the company reported loan growth, margin improvement, and stable asset quality in its banking business.

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Subadviser outlook

During this period of crisis and uncertainty, Invesco remains focused on understanding the macroeconomic pressures that are idiosyncratic to emerging markets. Nevertheless, Invesco is unwavering in its approach as bottom-up investors that focus on the long term, while avoiding tactical missteps. Fund management plans to continue to seek high-quality companies that they believe have durable long-term growth potential – supported by strong competitive positions, balance sheets, and cash flows – which Fund management believes will allow them to thrive in the post-COVID-19 world. As of September 30, 2021, the Fund had exposure to sectors and industries where Fund management sees the potential for dynamic change and real value being extracted – including e-commerce, cloud computing, internet services, health care, travel, and education.

46

GROWTH OF $10,000 INVESTMENT - Class R5

The graph above illustrates a representative class of the Fund’s historical performance for the past 10 fiscal years or since inception (for Funds lacking 10-year records) in comparison to its benchmark index, as well as one or more additional indexes, if applicable. The performance of other share classes will be greater than or less than the class depicted above.

Generally accepted accounting principles require adjustments to be made to the net assets of the Fund at period end for financial reporting purposes only, and as such, the total return based on the unadjusted net asset value per share may differ from the total return reported in the financial highlights.

Performance results in the graph and table are shown both with and without the imposition of the maximum applicable sales charge (if applicable) and reflect the percentage change in net asset value, including reinvestment of dividends and capital gains distributions. Investors should note that the Fund is a professionally managed mutual fund, while the index or indexes shown above are unmanaged, cannot be purchased directly, and, with the exception of any peer group index, do not incur expenses.

Performance data quoted in the graph and table represents past performance; past performance is not predictive of future results. The investment return and principal value of shares of the Fund fluctuate, so your shares, when sold, may be worth more or less than their original cost. Current performance of the Fund may be lower or higher than the performance quoted. The performance shown does not reflect the deduction of taxes, if any, that a shareholder would pay on Fund distributions or the redemption of Fund shares. Performance data current to the most recent month end may be obtained by calling 1-888-309-3539.

Investors should read the Fund’s prospectus with regard to the Fund’s investment objectives, risks, and charges and expenses in conjunction with these financial statements. Employee retirement benefit plans that invest plan assets in the Separate Investment Accounts (SIAs) may be subject to certain charges as set forth in their respective Plan Documents. Total return figures would be lower for the periods presented if they reflected these charges.

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Notes to Portfolio of Investments

Percentages are stated as a percent of net assets.

The accompanying notes are an integral part of the financial statements.

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The accompanying notes are an integral part of the financial statements.

49