Giant Geyser is a cone-type geyser in the Upper Geyser Basin of Yellowstone National Park in the United States. Giant Geyser is the namesake for the Giant Group of geysers, which includes Bijou Geyser, Giant Geyser, and Mastiff Geyser. Giant Geyser is also the namesake for the Giant Platform, a raised stone structure incorporating all these geysers. Giant is notable for its spectacular, but sporadic eruptions, as well as for its very large cone of geyserite, which stands about 12 feet tall.

On September 18, 1870 the Washburn-Langford-Doane Expedition entered the Upper Geyser Basin and observed geysers erupting. During their day and a half explorations, they named seven geysers in the basin, including Giant. Nathaniel P. Langford in his 1871 Scribner's account described the Giant.

"The Giant" has a rugged crater, ten feet in diameter on the outside, with an irregular orifice five or six feet in diameter. It discharges a vast body of water, and the only time we saw it in eruption the flow of water in a column five feet in diameter, and one hundred and forty feet in vertical height, continued uninterruptedly for nearly three hours. The crater resembles a miniature model of the Coliseum.