Lamar County is a county located in the U.S. state of Mississippi. As of the 2020 census, the population was 64,222. Its county seat is Purvis. Named for Confederate Lucius Quintus Cincinnatus Lamar, the county was carved out of Marion County to the west in 1904. Lamar County is part of the Hattiesburg, MS Metropolitan Statistical Area. It is largely a rural county, except for its northeast quarter.