Boysen State Park, State park and reservoir in Fremont County, US.

Boysen State Park surrounds the Boysen Reservoir, a 1951 impoundment of the north-flowing Wind River, in Fremont County, Wyoming. The park covers more than 35,000 acres at the south end of the Owl Creek Mountains, where the Wind River Canyon opens. Campgrounds, boat launches and a privately operated marina line the reservoir shore.

Asmus Boysen began building the first dam on the Wind River in 1908, together with a hydroelectric plant producing 710 kilowatts, mainly for nearby mines. A flood destroyed the plant in 1923 and power generation stopped; remnants of that dam remain visible. The present dam was finished in 1951 with a 15,000-kilowatt plant, and the park opened in 1956.

The park is named after Asmus Boysen, who began building the first dam and a 710-kilowatt hydroelectric plant here in 1908; the power served nearby mines until a flood destroyed the plant in 1923. Remains of that original dam can still be seen. The reservoir's game fish include walleye, sauger, perch, crappie, ling, rainbow, cutthroat and brown trout.

The park is open year-round, with reduced service in winter, and has three entrances: U.S. Highway 20 on the east side, U.S. Highway 26 on the south and Bass Lake Road on the west. Eleven campgrounds provide overnight stays, and an entrance fee is charged.

Asmus Boysen's original 1908 dam and its 710-kilowatt powerhouse supplied power mainly to nearby mines until a flood destroyed the plant in 1923; remnants of that first dam are still visible. The replacement dam, finished in 1951, generates 15,000 kilowatts.