St. Charles is a city in DuPage and Kane counties in the U.S. state of Illinois. It lies roughly 40 miles (64 km) west of Chicago on Illinois Route 64. Per the 2020 census, the population was 33,081. The official city slogan is "Pride of the Fox", after the Fox River that runs through the center of town. St. Charles is part of a tri-city area along with Geneva and Batavia, all western suburbs of similar size and socioeconomic condition.

Illinois Route 64 is an east–west state highway in Northern Illinois. Its western terminus is at the Iowa state line, connecting with U.S. Route 52 (US 52) and Iowa Highway 64 via the Dale Gardner Veterans Memorial Bridge at the Mississippi River west of Savanna. IL 64 then travels east through Mount Carroll, Lanark, Mount Morris, Oregon, Sycamore, Lily Lake, Saint Charles and the western suburbs of Chicago before terminating at U.S. Route 41 on the city's north side. IL 64 is 138.61 miles (223.07 km) long.