Boysen Reservoir, reservoir in Fremont County, Wyoming, United States

Boysen Reservoir is a reservoir on the Wind River in Fremont County, Wyoming, held back by Boysen Dam, an earth-fill dam built between 1947 and 1952 at the mouth of Wind River Canyon. The water lies at an elevation of 1,463 m, its rippled surface a muted reddish-brown, and a low strip of sparse dark vegetation and pale earth marks the far shore. The dam and much of the reservoir stand on the Wind River Indian Reservation.

Asmus Boysen built a first dam on the Wind River in 1908, on land leased from the Shoshone and Arapaho tribes. The present earth-fill dam was built between 1947 and 1952 at the mouth of Wind River Canyon, just upstream of that earlier structure. Its construction flooded the Billings–Casper railroad, so a new line was laid through a 1⅓-mile tunnel under the dam and around the reservoir's edges.

The reservoir takes its name from Asmus Boysen, who in 1908 built an earlier dam on the Wind River on land leased from the Shoshone and Arapaho tribes. The dam and much of the water lie within the Wind River Indian Reservation. Boysen State Park, run by Wyoming, surrounds the lake with 11 campgrounds, and Boysen Marina on the northeast shore rents boats and jet skis.

The area has a State Park with camping facilities, boat rental, and fishing supplies available at the marina. The shoreline offers multiple accessible spots to stop and rest, though some side roads are rough and require slow driving to reach quieter areas.

When the dam was built, the railroad line between Billings, Montana and Casper, Wyoming was flooded. A new track was laid starting near the dam, where a tunnel of about 1⅓ miles carries the rails under the dam and around the edge of the reservoir.