Big Sage Reservoir is an artificial lake in Modoc County, California and Modoc National Forest. Its waters are impounded by Big Sage Dam, which was completed in .
The lake discharges into Rattlesnake Creek, a tributary of the Pit River.
Big Sage Dam is an earthen dam long and high, with of freeboard. Hot Springs Valley Irrigation District owns the dam.
Facilities include a campground and boat ramp, both located just north of the dam.