Troy is a city in Madison County, Illinois, United States.

Its namesake in Lincoln County, Missouri, is also part of this MSA, making it (along with the two O'Fallons in Illinois and Missouri) one of the few pairs of like-named municipalities to be part of the same MSA.

Interstates 55 and 70 pass through the west side of Troy, with access from exits 17 and 18.

U.S. Route 40 passes through the south side of Troy, joining I-55 and I-70 at the western edge of the city and leading east 11 miles (18 km) to Highland.

Illinois Route 162 passes through the center of Troy as Center Street, Market Street, and Edwardsville Road, leading east 2.5 miles (4.0 km) to US 40 and west 4 miles (6 km) to the north part of Maryville.

The westernmost part of the city drains to Canteen Creek, which flows southwest to the Mississippi River valley at East St. Louis.