Mammoth Hot Springs, Thermal terraces in Yellowstone National Park, Wyoming, United States.

Mammoth Hot Springs is a hillside complex of travertine terraces in Yellowstone National Park, next to Fort Yellowstone. Hot water arrives underground from Norris Geyser Basin along a fault through limestone, cools to about 170 °F (80 °C) and deposits calcium carbonate, over two tons a day. The stepped basins hold still, clear water; algae tint the crust brown, orange, red and green, and Minerva Terrace is the best-known group.

The terraces and springs were officially documented in 1871 by Ferdinand V. Hayden's geological expedition, and prospector Harry Horr named the springs that same year. Fort Yellowstone was built here between 1891 and 1913; its garrison administered the park until the National Park Service took over on 30 August 1916, with final transfer of administration in 1918. The park is still run from Mammoth Hot Springs.

The name comes from the nearby sinter terraces and hot springs; the settlement itself holds only a restaurant, hotel, souvenir shop, gas station, chapel, post office and staff housing. Elk herds often lie on the warm travertine terraces in the morning and evening, and one herd has settled in the village, so some shops protect their flower boxes with grilles against grazing.

Visitors can explore two miles of elevated boardwalks winding through the terrace complex, offering safe and close views of the hot springs. The paths are well-maintained and allow visitors to choose shorter or longer routes depending on their comfort level.

The hot water feeding Mammoth rises from Norris Geyser Basin, travelling underground along a fault line through limestone roughly parallel to the Norris-to-Mammoth road. That limestone is the source of the calcium carbonate built into the terraces. Shallow circulation lets the superheated water cool to about 170 °F (80 °C) before it surfaces.