Coosa County was created by the Alabama legislature on 1832 Dec. 18, from lands included in the Creek Indian Treaty of Cusseta, 1832 Mar. 24. It was named for the Coosa River, which forms its western boundary. The word "Coosa" is believed to mean "cane-brake" in the Alibama-Kossati Indian dialect.

Coosa County lies in the east-central part of the state. It is bordered by Shelby, Talladega, Clay, Tallapoosa, Elmore, and Chilton counties. It currently comprises 675 square miles. A site on Albert Crumpler's plantation on Hatchemalega Creek was chosen as the county seat and given the name Lexington. In 1835 the name was changed to Rockford. Other towns and communities include Equality, Nixburg, and Goodwater.