The discovery was made on Tuesday at Abyss Pool and resulted in the temporary closure of the West Thumb Geyser Basin. It has since been reopened, according to the Associated Press.
Park officials had no further information on the investigation as of Thursday.
Abyss Pool is located in the West Thumb of Yellowstone Lake. The hot spring is 53-feet deep and reaches water temperatures of 140 F (60C), park officials told the AP.
At 140F, a human's skin would suffer serious burns in approximately three seconds.
Yellowstone's hot springs and geysers have caused fatalities in the past when visitors have ventured too close and fallen into the superheated waters.
Since 1870, 22 people have died in the park after falling into the pools and geysers. One of the most recent deaths, in 2016, involved a man trying to soak in a geyser in the park. The man fell into the water and was boiled to death. His body dissolved in the superheated waters before it could be recovered by park staff.
Its unclear if the foot found in the pool is the result of a visitor accident or if it ended up there by some other means.
Yellowstone National Park had numerous signs posted near the geysers and hot springs warning visitors to stay on the trail, both for their protection and to protect the ecosystems surrounding the geysers.
The park sells a book in its giftshop titled Death in Yellowstone, that collects the stories of individuals who have died in the park. The first chapter is titled “Death by Hot Water.”