Stephens County is a county located in the U.S. state of Oklahoma. As of the 2020 census, its population was 42,848. Its county seat is Duncan. The county was created at statehood, partly from the Chickasaw Nation in Indian Territory and partly from Comanche County in Oklahoma Territory. It was named for Texas politician John Hall Stephens.

Duncan is a city and 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. The Chisholm Trail passed to the east of Duncan prior to the town's founding, which is home to the Chisholm Trail Heritage Center.