Rockwall County is the a county located in the U.S. state of Texas. At 129 square miles, Rockwall County has the smallest area of any county in Texas. As of the 2014 census, its population was 87,809. Its county seat is Rockwall. The county and city are named for a wall-like subterranean rock formation that runs throughout the county.Rockwall County is part of the Dallas-Fort Worth-Arlington, TX Metropolitan Statistical Area.