Columbia is a city mainly in Monroe County with a small portion in St. Clair County in the U.S. state of Illinois, about 12 miles (19 km) south of St. Louis, Missouri. The population was 9,707 at the 2010 census.
Columbia is located at 38°26′57″N 90°12′31″W (38.449172, -90.208500).
According to the 2010 census, Columbia has a total area of 10.476 square miles (27.13 km2), of which 10.41 square miles (26.96 km2) (or 99.37%) is land and 0.066 square miles (0.17 km2) (or 0.63%) is water.
Columbia is bounded on the west by the Mississippi River. Running in a diagonal line just northeast of Columbia is the border of St. Clair County, beyond which lies Dupo to the north and Millstadt to the east. To the south is Waterloo. Waterloo and Columbia are expected to grow right up to the borders of the other, in effect extending St. Louis's urban sprawl from South St.