Grotto Geyser, Geyser in Upper Geyser Basin, Yellowstone National Park, Wyoming, US

Grotto Geyser is a cone-type geyser in the Upper Geyser Basin of Yellowstone National Park, Wyoming. Its cone stands nearly 8 feet (2.4 m) high and has an odd, cave-like shape because sinter accumulated over dead tree stumps as the geyser erupted. The cone is full of large, sinuous apertures through which water is discharged.

On September 18, 1870, members of the Washburn-Langford-Doane Expedition entered the Upper Geyser Basin and, within a day and a half, named seven geysers, Grotto among them. Nathaniel P. Langford described the crater in his 1871 Scribner's account, telling how a companion crawled through one of its sinuous apertures to the discharging orifice and later saw boiling water shoot sixty feet high through it.

The name of the geyser comes from the cave-like shape of its cone, built up over time by mineral deposits. Visitors can watch water shoot from several openings at once, which sets it apart from the more symmetrical geysers nearby.

Grotto Geyser stands beside the boardwalk of the Upper Geyser Basin, a walk from the Old Faithful area. Its eruptions come roughly every eight hours, and the wait is longer after a long eruption. Short-mode eruptions last one to two hours; long-mode ones run six to twelve hours, so check current activity before settling in.

Eruptions at Grotto Geyser can last for several hours at a time, which is unusually long compared to most geysers. During these extended bursts, water shoots simultaneously from different openings and in different directions.