Lamar is an unincorporated community in Benton County, Mississippi, United States. It is located along Mississippi Highway 7 in western Benton County. Although it is unincorporated, Lamar has a post office, with the ZIP code of 38642.. The nearly abandoned Mississippi Central Railroad runs through Lamar, and is only used rarely for freight trafficking. Lamar shares its ZIP code with the nearby community of Slayden.