Herrin is a city in Williamson County, Illinois. The population was 12,352 at the 2020 census. The city is part of the Marion-Herrin Micropolitan Area and is a part of the Carbondale-Marion-Herrin, Illinois Combined Statistical Area with 123,272 residents, the sixth most populous Combined statistical area in Illinois.

Williamson County is a county in Southern Illinois. At the 2020 census, it had a population of 67,153. The largest city and county seat is Marion.