| Cover | Aug. 03, 2021 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Amendment Flag | false |
| Document Period End Date | Aug. 3, 2021 |
| Entity File Number | 001-33076 |
| Entity Registrant Name | WILLDAN GROUP, INC. |
| Entity Central Index Key | 0001370450 |
| Entity Tax Identification Number | 14-1951112 |
| Entity Incorporation, State or Country Code | DE |
| Entity Address, Address Line One | 2401 East Katella Avenue |
| Entity Address, Address Line Two | Suite 300 |
| Entity Address, City or Town | Anaheim |
| Entity Address, State or Province | CA |
| Entity Address, Postal Zip Code | 92806 |
| City Area Code | 800 |
| Local Phone Number | 424-9144 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Title of 12(b) Security | Common Stock, par value $0.01 per share |
| Trading Symbol | WLDN |
| Security Exchange Name | NASDAQ |
| Entity Emerging Growth Company | false |
