Geysir (sometimes known as The Great Geysir) is a geyser in Iceland. It is in the Haukadalur valley, Iceland. It is the oldest known geyser and one of the world's most impressive ones. The English word geyser comes from Geysir. Geysir is on the slopes of Laugarfjall hill at . The Strokkur geyser is about 400 metres south of Geysir.
Eruptions at Geysir can send boiling water up to 60 metres in the air. However, eruptions may be infrequent. In the past, they have in the past stopped altogether for years at a time.