Saint Clair is a city in Franklin County, Missouri, United States. The population was 4,472 at the 2010 census.
Saint Clair is located between the Meramec River to the southeast and the Bourbeuse River to the northwest. Interstate 44 passes along the northwest side of the city and Missouri Routes 30 and 47 both pass through the city. Union is approximately six miles north along Route 47 and Stanton is about eight miles to the southwest on I-44. Parkway is on Route 30, just southeast of the city.
According to the United States Census Bureau, the city has a total area of 3.71 square miles (9.61 km2), of which 3.70 square miles (9.58 km2) is land and 0.01 square miles (0.03 km2) is water.
An early variant name was "Travelers Repose". Saint Clair was platted in 1859 when the railroad was extended to that point. The present name is after a railroad official with the surname St. Clair.
The International Shoe Company Building and Panhorst Feed Store are listed on the National Register of Historic Places.
As of the census of 2010, there were 4,724 people, 1,908 households, and 1,136 families living in the city. The population density was 1,276.8 inhabitants per square mile (493.0/km2). There were 2,142 housing units at an average density of 578.9 per square mile (223.5/km2).