Duncan is a city in and the county seat of Stephens County, Oklahoma, United States. Its population was 22,310 at the 2020 census. Centrally located in Stephens County, Duncan became the county seat after Oklahoma achieved statehood in 1907. Oil wells opened in Stephens County in 1918 and led to rapid development. Cotton was a dominant crop until the Dust Bowl brought its decline, but cattle remain an important part of the economy.