Lewis Lake is located in the U. S. state of Wyoming in the southern part of Yellowstone National Park, about 4 miles (6.4 km) southeast of Shoshone Lake, and approximately 10 miles (16 km) southwest of Yellowstone Lake.

The primary outflow of Lewis Lake is also the Lewis River, continuing south to join the Snake River near the southern boundary of Yellowstone National Park.

A few small creeks also feed Lewis Lake.

There are several hot springs emptying into the lake on its northwestern shore and two more hot springs at the lake's southern end where it empties into the Lewis River.

A campground is located on the lake's southeast shore.