Casper is a city in Natrona County, Wyoming, United States. The population was 49,644 at the 2000 census. It is a medium-sized city in east-central Wyoming on Interstate 25. It is the county seat of Natrona County^GR6 and the second largest city in Wyoming.

The city sits at the foot of Casper Mountain, the north end of the Laramie Mountain Range along the North Platte River. The city was site of Fort Caspar during the days of the Oregon Trail, which passed along the North Platte through the town. Although small by national standards, its large size within the region make it a banking and commerce center. Since the discovery of crude oil in the region in the 1890s it has been a center of the regional petroleum industry. The famous Salt Creek Oil Field was first drilled in 1889 approximately 40 miles (64 km) north of Casper, and the first refinery in Casper was built in 1895.

With the development of the Wyoming coal fields in recent decades, it has continued its role as an important center of the energy industry. Casper and the rest of Wyoming is served by the Casper Star-Tribune, a newspaper with statewide circulation. Casper is also the site of Casper College. The city is home to the Casper Troopers, a drum and bugle corps in Drum Corps International.

Casper was the hometown of Matthew Shepard, a University of Wyoming student who was murdered in October 1998 in an apparent anti-gay hate crime that received international media attention. In January 2005 Shepard's former high school classmate Guy V. Padgett III was elected mayor of Casper, becoming the first openly gay elected official in Wyoming history. JM J. Bullock, a famous gay actor was also born in the town.