According to the United States Census Bureau, the city has a total area of 3.61 square miles (9.35 km2), of which 2.93 square miles (7.59 km2) is land and 0.68 square miles (1.76 km2) is water. The city is located along the St. Clair River.
Saint Clair can also be considered as in the Blue Water area, a subregion of the Thumb.
The City of St. Clair has much upscale housing along the St. Clair River. The Saint Clair area attracts many tourists from Metro Detroit. The city also boasts the longest freshwater boardwalk in the world.