Anadolu Agency said 22 of the militants detained are foreign nationals, but the report didn't provide details on their nationalities. Seven of them are Turkish citizens.
The suspects were detained in a police swoop targeting 20 different addresses around Istanbul, the agency reported.
Police seized ISIL documents, digital material and one firearm during the raids, AP reported.
It said the suspects are believed to have fought with ISIL in Syria and were making plans to return to the conflict-torn country.
Turkey has been hit by a series of deadly attacks carried out by ISIL or Kurdish militants and has stepped up anti-terrorism operations across the country. About 750 people with alleged ISIL links were detained in a major police sweep earlier this year.
ISIL claimed responsibility for a New Year's mass shooting at an Istanbul nightclub that killed 39 people.