| Cover | Apr. 30, 2021 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Amendment Flag | false |
| Document Period End Date | Apr. 30, 2021 |
| Entity File Number | 001-33287 |
| Entity Registrant Name | Information Services Group Inc. |
| Entity Central Index Key | 0001371489 |
| Entity Tax Identification Number | 20-5261587 |
| Entity Incorporation, State or Country Code | DE |
| Entity Address, Address Line One | 2187 Atlantic Street |
| Entity Address, City or Town | Stamford |
| Entity Address, State or Province | CT |
| Entity Address, Postal Zip Code | 06902 |
| City Area Code | 203 |
| Local Phone Number | 517-3100 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Title of 12(b) Security | Shares of Common Stock, $0.001 par value |
| Trading Symbol | III |
| Security Exchange Name | NASDAQ |
| Entity Emerging Growth Company | false |
