“The child took off running from the trail, slipped and then fell into a small thermal feature,” park officials said.
The accident happened near Midway Geyser Basin on the park’s western side.
The toddler was flown to the burns unit at Eastern Idaho Regional Medical Center, with severe damage to the lower body and back.
The child’s identity, condition and whether the child was a boy or girl were not disclosed.
“The ground in hydrothermal areas is fragile and thin, and there is scalding water just below the surface,” the parks service said in a statement.
“Visitors must always remain on boardwalks and trails and exercise extreme caution around thermal features.”
It was the second injury in a Yellowstone thermal feature this year.
A woman backing up and taking photos fell into a hot spring or fumarole near Old Faithful Geyser in May.
The woman had entered the park illegally while it was closed due to the coronavirus.
Injuries amid the boiling springs are relatively rare, but not unheard of.
In September 2019, a man suffered severe burns after falling into thermal water near the cone of Old Faithful Geyser; two years earlier, in June 2017, a man sustained severe burns after falling in a hot spring in the Lower Geyser Basin.
The most recent fatality was in June 2016, when a man left the boardwalk and died after slipping into a hot spring in Norris Geyser Basin.
And in August 2000, one person died and two people received severe burns from falling into a hot spring in the Lower Geyser Basin.