A steam devil is a small, weak whirlwind over water (or sometimes wet land) that has drawn fog into the vortex, thus rendering it visible. They form over large lakes and oceans during cold air outbreaks while the water is still relatively warm, and can be an important mechanism in vertically transporting moisture.

Smaller steam devils and steam whirls can form over geyser basins even in warm weather because of the very high water temperatures. Although steam devils are generally quite a rare phenomenon, hot springs in Yellowstone Park produce them on a daily basis.

These phenomena have only been reported and studied since the 1970s.