| Document And Entity Information | Mar. 26, 2021 |
| Document Information [Line Items] | |
| Entity, Registrant Name | RBB BANCORP |
| Document, Type | 8-K |
| Document, Period End Date | Mar. 26, 2021 |
| Entity, Incorporation, State or Country Code | CA |
| Entity, File Number | 001-38149 |
| Entity, Tax Identification Number | 27-2776416 |
| Entity, Address, Address Line One | 1055 Wilshire Blvd., 12th Floor |
| Entity, Address, City or Town | Los Angeles |
| Entity, Address, State or Province | CA |
| Entity, Address, Postal Zip Code | 90017 |
| City Area Code | 213 |
| Local Phone Number | 627-9888 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Title of 12(b) Security | Common Stock |
| Trading Symbol | RBB |
| Security Exchange Name | NASDAQ |
| Entity, Emerging Growth Company | true |
| Entity, Ex Transition Period | false |
| Amendment Flag | false |
| Entity, Central Index Key | 0001499422 |
