Dallam County is a county located in the northwestern corner of the U.S. state of Texas. As of the 2010 census, its population was 6,703. Its county seat is Dalhart. The county was founded in 1876 and later organized in 1891. It is named for James Wilmer Dallam, a lawyer and newspaper publisher.Dallam is the northernmost of the ten Texas counties that from 1885–1912 constituted the legendary XIT Ranch.