Rankin County is a county located in the U.S. state of Mississippi. The western border of the county is formed by the Pearl River. As of the 2010 census, the population was 141,617, making it the fourth-most populous county in Mississippi. The county seat is Brandon. The county is named in honor of Christopher Rankin, a Mississippi Congressman who served from 1819 to 1826. Rankin County is part of the Jackson Metropolitan Statistical Area.