| Cover | Nov. 11, 2020 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Amendment Flag | false |
| Document Period End Date | Nov. 11, 2020 |
| Entity File Number | 0-19655 |
| Entity Registrant Name | TETRA TECH,INC. |
| Entity Central Index Key | 0000831641 |
| Entity Tax Identification Number | 95-4148514 |
| Entity Incorporation, State or Country Code | DE |
| Entity Address, Address Line One | 3475 East Foothill Boulevard |
| Entity Address, City or Town | Pasadena |
| Entity Address, State or Province | CA |
| Entity Address, Postal Zip Code | 91107 |
| City Area Code | 626 |
| Local Phone Number | 351-4664 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Title of 12(b) Security | Common Stock, $0.01 par value |
| Trading Symbol | TTEK |
| Security Exchange Name | NASDAQ |
| Entity Emerging Growth Company | false |
