| Cover | Jan. 25, 2021 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Document Period End Date | Jan. 25, 2021 |
| Entity Registrant Name | PACIFIC MERCANTILE BANCORP |
| Entity Incorporation State | CA |
| Entity File Number | 0-30777 |
| Entity Tax Identification Number | 33-0898238 |
| Entity Address, Street Address | 949 South Coast Drive |
| Entity Address, City | Costa Mesa |
| Entity Address, State | CA |
| Entity Address, Postal Zip Code | 92626 |
| City Area Code | 714 |
| Local Phone Number | 438-2500 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Title of each class | Common Stock, without par value |
| Trading Symbol(s) | PMBC |
| Name of each exchange on which registered | NASDAQ |
| Entity Emerging Growth Company | false |
| Entity Central Index Key | 0001109546 |
| Amendment Flag | false |
