Note 17 - Regulatory Matters - Summary of Company and Bank Continue to Exceed Regulatory Capital Minimum Requirements and Bank Continues to Exceed Well Capitalized Standards (Details) $ in Thousands	Mar. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)	Jan. 01, 2015	Dec. 31, 2014
Banking Regulation, Tier 1 Risk-Based Capital Ratio, Capital Adequacy, Minimum			0.060	0.040
RBB Bancorp [Member]
Tier 1 Leverage Ratio, Amount	$ 415,013	$ 410,134
Tier 1 Leverage Ratio, Ratio	0.0990	0.1021
Tier 1 Leverage Ratio, Minimum Required for Capital Adequacy Purposes Amount	$ 167,665	$ 160,642
Tier 1 Leverage Ratio, Minimum Required for Capital Adequacy Purposes Ratio	0.040	0.040
Common Equity Tier 1 Risk-Based Capital Ratio, Amount	$ 400,457	$ 395,632
Common Equity Tier 1 Risk-Based Capital Ratio, Ratio	0.1412	0.1486
Common Equity Tier 1 Risk-Based Capital Ratio, Minimum Required for Capital Adequacy Purposes Amount	$ 127,617	$ 119,841
Common Equity Tier 1 Risk-Based Capital Ratio, Minimum Required for Capital Adequacy Purposes Ratio	4.50%	4.50%
Tier 1 Risk-Based Capital Ratio, Amount	$ 415,013	$ 410,134
Tier 1 Risk-Based Capital Ratio, Ratio	0.1463	0.1540
Tier 1 Risk-Based Capital Ratio, Minimum Required for Capital Adequacy Purposes Amount	$ 170,156	$ 159,788
Banking Regulation, Tier 1 Risk-Based Capital Ratio, Capital Adequacy, Minimum	0.060	0.060
Total Risk-Based Capital Ratio, Amount	$ 622,644	$ 616,440
Total Risk-Based Capital Ratio, Ratio	0.2196	0.2315
Total Risk-Based Capital Ratio, Minimum Required for Capital Adequacy Purposes Amount	$ 226,875	$ 213,051
Total Risk-Based Capital Ratio, Minimum Required for Capital Adequacy Purposes Ratio	0.080	0.080
Royal Business Bank [Member]
Tier 1 Leverage Ratio, Amount	$ 514,298	$ 499,325
Tier 1 Leverage Ratio, Ratio	0.1229	0.1245
Tier 1 Leverage Ratio, Minimum Required for Capital Adequacy Purposes Amount	$ 167,428	$ 160,418
Tier 1 Leverage Ratio, Minimum Required for Capital Adequacy Purposes Ratio	0.040	0.040
Tier 1 Leverage Ratio, Minimum Required Plus Capital Conservation Buffer Fully Phased-In Amount	$ 209,284	$ 200,523
Tier 1 Leverage Ratio, Minimum Required Plus Capital Conservation Buffer Fully Phased-In Ratio	5.00%	5.00%
Common Equity Tier 1 Risk-Based Capital Ratio, Amount	$ 514,298	$ 499,325
Common Equity Tier 1 Risk-Based Capital Ratio, Ratio	0.1815	0.1880
Common Equity Tier 1 Risk-Based Capital Ratio, Minimum Required for Capital Adequacy Purposes Amount	$ 127,522	$ 119,550
Common Equity Tier 1 Risk-Based Capital Ratio, Minimum Required for Capital Adequacy Purposes Ratio	4.50%	4.50%
Common Equity Tier 1 Risk-Based Capital Ratio, Minimum Required Plus Capital Conversation Buffer Fully Phased-In, Amount	$ 184,199	$ 172,684
Common Equity Tier 1 Risk-Based Capital Ratio, Minimum Required Plus Capital Conversation Buffer Fully Phased-In, Ratio	0.065	0.065
Tier 1 Risk-Based Capital Ratio, Amount	$ 514,298	$ 499,325
Tier 1 Risk-Based Capital Ratio, Ratio	0.1815	0.1880
Tier 1 Risk-Based Capital Ratio, Minimum Required for Capital Adequacy Purposes Amount	$ 170,030	$ 159,401
Banking Regulation, Tier 1 Risk-Based Capital Ratio, Capital Adequacy, Minimum	0.060	0.060
Tier 1 Risk-Based Capital Ratio, Minimum Required Plus Capital Conversation Buffer Fully Phased-In, Amount	$ 226,706	$ 212,534
Tier 1 Risk-Based Capital Ratio, Minimum Required Plus Capital Conversation Buffer Fully Phased-In, Ratio	8.00%	8.00%
Total Risk-Based Capital Ratio, Amount	$ 548,777	$ 532,544
Total Risk-Based Capital Ratio, Ratio	0.1937	0.2005
Total Risk-Based Capital Ratio, Minimum Required for Capital Adequacy Purposes Amount	$ 226,706	$ 212,534
Total Risk-Based Capital Ratio, Minimum Required for Capital Adequacy Purposes Ratio	0.080	0.080
Total Risk-Based Capital Ratio, Minimum Required Plus Capital Conversation Buffer Fully Phased-In, Amount	$ 283,383	$ 265,668
Total Risk-Based Capital Ratio, Minimum Required Plus Capital Conversation Buffer Fully Phased-In, Ratio	10.00%	10.00%