is a city in Stephens County, Oklahoma, United States. The population was 22,505 at the 2000 census. It is the county seat of Stephens County.
Duncan is located at (34.516619, -97.949377). The town is situated approximately 30 miles East of Lawton, Oklahoma and 90 miles Southwest of Oklahoma City, Oklahoma along US Highway 81.
According to the United States Census Bureau, the city has a total area of 119.1 km² (46.0 mi²). 100.5 km² (38.8 mi²) of it is land and 18.7 km² (7.2 mi²) of it (15.67%) is water.