Old Faithful, Cone geyser in Yellowstone National Park, Wyoming, United States

Old Faithful is a cone geyser in the Upper Geyser Basin of Yellowstone National Park, Wyoming. Its vent opens 0.6 by 1 metre and is lined with coarse siliceous sinter, feeding a narrow jet that rises from a cracked mound of pale mineral crust. Eruptions throw 14,000 to 32,000 litres of water 32 to 56 metres high for 1.5 to 5 minutes.

On 18 September 1870 the Washburn–Langford–Doane Expedition entered the Upper Geyser Basin and saw Old Faithful erupt nine times; Langford recorded columns of 90 to 125 feet lasting 15 to 20 minutes, and the party gave it its name. In 1882 General Sheridan's men found that linen and cotton left in the crater came out washed, while woollen clothes were torn to shreds.

Members of the Washburn–Langford–Doane Expedition gave the name in 1870 after watching it spout nine times in one stay; Langford called it "Old Faithful" for its regular intervals. In the park's early years the crater served as a laundry: garments left in it during quiescence came out washed, though wool was torn to shreds.

The next eruption is forecast at the Visitor Center and through the National Park Service app, using a formula by George D. Marler based on the last interval and duration. Forecasts are usually accurate to about five minutes, but can be off by 20 after a long prelude of small splashes. The geyser sits at 2,240 m, so expect cold and changeable weather.

The eruption conduit narrows to just 10.5 cm at a point 7.31 m below the surface, above an upper chamber; probes lowered in 1983–1994 measured 118 °C water at 22 m depth. The geyser sits at 2,240 m above sea level.