Bighorn Lake is a reservoir behind Yellowtail Dam within the Bighorn Canyon National Recreation Area, located in northern Wyoming and extending into southern Montana.
The reservoir is <span style="white-space:nowrap">40 mi (64 km)</span> south of Billings, Montana. It stretches the entire <span style="white-space:nowrap">72 mi (115 km)</span> length of the Bighorn Canyon at full pool. The Lake was created by the 1965 construction of Yellowtail Dam on the Bighorn River, near Fort Smith, Montana.
Bighorn Lake has a surface area of 5574 acres, because of the dam, streamflow in the river is relatively stable with little daily fluctuation. Bighorn Lake supports a warmwater fishery.