Wyoming Highway 91 is a 23.10-mile (37.2 km) east-west (southwest-northeast) Wyoming State Road located in south-central Converse County southwest of Douglas.

Wyoming Highway 91 begins its western end at Converse County Route 11 southwest of La Prele and near the Medicine Bow National Forest boundary. WYO 91 changes to Converse County Route 24 south of there. Highway 91 proceeds northeast as Cold Springs Road and through the community of La Prele. As Highway 91 nears Douglas, WYO 91 turns north and meets the eastern terminus of Wyoming Highway 96 at 20.08 miles (32.32 km). WYO 96 heads west as WYO 91 turns east toward Douglas. Almost at the Douglas city limits, Interstate 25 comes into view to the north and is paralleled by Highway 91 to its end. WYO 91 reaches its eastern end at Wyoming Highway 94 in Douglas, south of Interstate 25 exit 140.

Wyoming Highway 91 was first designated to a stretch of present day Wyoming Highway 89 in Lincoln County.

According to the original map of Wyoming State Highways in 1924, present day Wyoming Highway 89 was numbered Wyoming Highway 65. With the introduction of U.S. routes in 1926; The routing between Cokeville and Star Valley was renamed Wyoming Highway 89, while the section south of Sage remained Wyoming Highway 65. WYO 89 was numbered in hopes that U.S. Route 89 (which had a northern terminus in central Utah at that time) would be extended north into Wyoming. In 1936, US 89 was extended into Wyoming along old Wyoming Highway 287. Wyoming Highway 89 was then renumbered to Wyoming Highway 91 to avoid confusion between the two highways, while Wyoming Highway 65 kept its original number. However, within a few years, U.S. 89 was rerouted to its current route in Utah and Idaho, and US Route 189 was created along old WYO 287. As a result Wyoming Highway 89 was redesignated over both WYO 65 and WYO 91.

The entire route is in Converse County.