Opal Pool, Hot spring in Midway Geyser Basin, Yellowstone National Park, Wyoming, US.

Opal Pool is a hot spring in Midway Geyser Basin, Yellowstone National Park, Wyoming, at 2,210 m elevation. In quiet periods it looks like a small lake at about 56 °C; when it erupts, a single burst throws water 20–80 feet high, followed by smaller splashes for about a minute.

Opal Pool's first recorded eruption was in 1947, with further eruptions in 1949, 1952 and 1953 before activity ceased. Eruptions resumed in 1979 and have occurred at least once in most years since. In 2005 the pool drained completely and refilled by 2008.

The pool takes its name from the opalescent shimmer visible in its water under certain light angles. Visitors notice this color shift especially when sunlight hits the water from different directions.

Access is via paved walkways that start directly from the Midway Geyser Basin visitor area. It is important to stay on marked paths and follow park safety rules since the water is extremely hot.

The basin drained completely in 2005 and refilled only in 2008, a rare transformation for a geothermal spring. After this emptying, the water now displays a green color and eruptions can shoot up to 80 feet high.