Grand Geyser is a Fountain Geyser in the Upper Geyser Basin of Yellowstone National Park in the United States. It is the tallest predictable geyser known. It was named by Dr. F.V. Hayden in 1871.
Grand's fountain reaches a height of as much as 200 feet (61 m), with a duration of 9 to 12 minutes. Its eruptions occur in a series of 1 to 4 (rarely as many as 5) bursts.
Grand erupts every 7 to 15 hours. It belongs to the Grand Group (or Grand Geyser Complex), and its eruption is connected to those of the other geysers in the group, especially the adjacent Vent Geyser and Turban Geyser.
For a few hours before an eruption by Grand, Turban Geyser erupts for a duration of five minutes about every 20 minutes. Grand's eruption begins within 1 to 2 minutes of one of Turban's eruptions with Vent Geyser erupting shortly after Grand. On occasions, Grand will stop erupting after 8 minutes only to restart with a taller fountain a minute or two later. Grand has been known to rarely experience over half a dozen restarts in a single eruption period. After Grand ceases its eruption, Vent and Turban continue erupting for another hour.