Isa Lake, Lake in Yellowstone National Park, United States

Isa Lake is a mountain lake in Yellowstone National Park situated directly on the Continental Divide at an elevation of about 8,300 feet (2,530 meters). It sits between the Old Faithful and West Thumb geyser basins along the main park roadway.

Hiram M. Chittenden became the first known European to sight the lake in 1891, while searching for the best routes connecting Old Faithful and the West Thumb Geyser Basin. He named it after Miss Isabel Jelke, from Cincinnati, though it is not clear why.

The Yellowstone region where Isa Lake stands has been home to multiple Indigenous Nations, including the Nimíipuu, Absaroke, and Shoshone for over 11,000 years.

The lake sits right along the main road between Old Faithful and West Thumb and is easily accessible by car. The area remains open year-round, though snow and ice in winter can affect driving conditions.

Isa Lake is one of the few natural lakes in the world that drain to two different oceans; the other cited example is Wollaston Lake. Its east side reaches the Pacific via the Lewis River, its west side the Gulf of Mexico via the Firehole River. The great yellow pond-lily grows in it.