Saint Clair is a city in Franklin County, Missouri, United States. According to the United States Census Bureau, the city has a total area of 3.1 square miles (8.0 square kilometers), of which, 3.1 square miles (8.0 square kilometers) of it is land and 0.32% is water. William Kerr founded the city. It is named after a railroad engineer surnamed Saint Clair. The city was originally known as Travelers Repose. Today it has the distinction of being one of the towns along the Historic Route 66. It is also well known in the area as having two water towers, facetiously labeled "Hot" and "Cold," respectively.