Troy is a city in Madison County, Illinois, United States. The population was 10,960 at the 2020 census, up from 9,888 in 2010.

Madison County is a county in the U.S. state of Illinois. It is a part of the Metro East in southern Illinois. According to the 2020 census, it had a population of 264,776, making it the eighth-most populous county in Illinois and the most populous in the southern portion of the state. The county seat is Edwardsville, and the largest city is Granite City.