File:Thermal image of Green Spring (8 July 2016) 1.jpg

English: The temperature scale is in degrees Fahrenheit.

Green Spring is an irregularly-shaped, moderately large hot spring in the Black Sand Basin of Yellowstone's Upper Geyser Basin. It is about 18 meters across from east to west and about 18 meters in its longest north-to-south dimension. It appears to have two vents. One vent is located in the north-central portion of the pool, near the edge of its northern geyserite shelf. Another is located in the southeastern part of the pool. Runoff channels drain to the southwest toward Iron Spring Creek.

Occasional Green Spring eruptions have been reported over the decades, from the 1930s to the 1980s. Eruptions were also observed in 2008. Reported activity in 2016 includes roiling and two foot-tall bulging bursts from the northern vent that result in overflow beyond the northern border of the pool (for example, see above photo).

In September 1998, researchers measured a temperature of ~77° Celsius (= 170° Fahrenheit) and a pH of 8.3. The spring's temperature has varied over the decades - it was formerly cool enough to have green algal growth, which was the inspiration for the name "Green Spring". When the spring is hotter, it has bluish-colored water and the algae are knocked out.