Duncan
- Country: United States
- State: Oklahoma
- Population: 22,985 (2016)
- Latitude: 34° 30.1' N (34.5023°)
- Longitude: 97° 57.5' W (-97.95781°)
- Elevation: 342 m/1,123 ft. above sea level
- Time zone: America/Chicago (UTC -06:00); DST observed (UTC -05:00)