Delta County is a county located in the U.S. state of Texas. As of the 2020 census, its population was 5,230. Its county seat and largest city is Cooper. The county was founded in 1870 and is named for its triangular shape, which resembles the Greek letter delta. Meanders of two forks of the Sulphur River formed its northern and southern boundaries and meet at its easternmost point. Delta County was one of 19 prohibition, or entirely dry, counties in the state of Texas.