| Document and Entity Information | Oct. 28, 2021 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Amendment Flag | false |
| Document Period End Date | Oct. 28, 2021 |
| Entity Registrant Name | PCSB Financial Corporation |
| Entity Central Index Key | 0001691337 |
| Entity Emerging Growth Company | true |
| Entity Ex Transition Period | true |
| Title of 12(b) Security | Common Stock, $0.01 par value per share |
| Trading Symbol | PCSB |
| Security Exchange Name | NASDAQ |
| Entity File Number | 001-38065 |
| Entity Incorporation, State or Country Code | MD |
| Entity Tax Identification Number | 81-4710738 |
| Entity Address, Address Line One | 2651 Strang Blvd |
| Entity Address, Address Line Two | Suite 100 |
| Entity Address, City or Town | Yorktown Heights |
| Entity Address, State or Province | NY |
| Entity Address, Postal Zip Code | 10598 |
| City Area Code | 914 |
| Local Phone Number | 248-7272 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
