Duncan
- County:
- Stephens County
- County Seat:
- Yes
- Area (mi²):
- 42.94
- State:
- Oklahoma
Duncan is a city located in Stephens County, Oklahoma. Duncan has a 2026 population of 23,174. It is also the county seat of Stephens County. Duncan is currently declining at a rate of -0.1% annually but its population has increased by 2.3% since the most recent census, which recorded a population of 22,653 in 2020.