Duncan is a city in Stephens County, Oklahoma, United States. The population was 23,431 at the 2010 census. It is the county seat of Stephens County.
The official birthdate of the town is considered to be when the first train arrived there on June 27, 1892. Statehood in 1907 turned the town into the county seat and a number of oil wells in Stephens County led to Duncan's rapid development in 1918.
The Chisholm Trail passed to the east of Duncan prior to the town's founding. An estimated 9,800,000 Longhorn cattle were herded up the trail between Texas and Abilene, Kansas during its existence. After learning that an extension of the Chicago, Rock Island and Pacific Railroad was being built from Kansas to Texas, Scotsman William Duncan brought his wife, parents, and other relatives and created a trading post situated at the intersection of the north-south Chisholm Trail and the east-west military passage between Fort Arbuckle and Fort Sill. The first train arrived on June 27, 1892; that date is considered the official birthdate of the town.