Arnett is a town in and the county seat of Ellis County, Oklahoma, United States. The population was 524 at the 2010 census.
The townsite came about at the junction of local trading routes. A post office was established at the townsite in 1902, with William G. Brown as the postmaster. Brown is said to have named the post office Arnett after A. S. Arnett, Brown's minister from Fayetteville, West Virginia.
Arnett is located at 36°8′4″N 99°46′15″W / 36.13444°N 99.77083°W / 36.13444; -99.77083 (36.134368, -99.770826) at an elevation of 2,447 feet (746 m). It lies approximately 9 miles (15 km) north-northeast of the Canadian River in the rolling Red Plains region of the Great Plains. Located in northwestern Oklahoma at the junction of U.S. Route 60 and U.S. Route 283, Arnett is 131 miles (210 km) west-northwest of Oklahoma City, 135 miles (217 km) northeast of Amarillo, and 172 miles (277 km) southwest of Wichita.