| Document and Entity Information | Feb. 24, 2021 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Amendment Flag | false |
| Document Period End Date | Feb. 24, 2021 |
| Entity Registrant Name | Timberland Bancorp Inc |
| Entity Incorporation, State or Country Code | WA |
| Entity File Number | 000-23333 |
| Entity Tax Identification Number | 91-1863696 |
| Entity Address, Address Line One | 624 Simpson Avenue |
| Entity Address, City or Town | Hoquiam |
| Entity Address, State or Province | WA |
| Entity Address, Postal Zip Code | 98550 |
| City Area Code | 360 |
| Local Phone Number | 533-4747 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Entity Emerging Growth Company | false |
| Entity Central Index Key | 0001046050 |
| Title of 12(b) Security | Common Stock, par value $.01 per share |
| Trading Symbol | TSBK |
| Security Exchange Name | NASDAQ |
