Hydrothermal vents are fissures in a planet's surface from which geothermally heated water issues. Hydrothermal vents are commonly found in places that are also volcanically active, where hot magma is relatively near the planet's surface.

Earth is both geologically active and has large amounts of water on its surface, and so has an abundance of hydrothermal vents. Common types include hot springs, fumaroles, geysers and black smokers. Black smokers are probably the most common, but due to their inaccessible location on ocean floors they have only recently been studied.

The water that issues from hydrothermal vents consists mostly of ground water that has percolated down into hot regions from the surface, but it also commonly contains some portion of virgin water that originated deep underground and is only now surfacing for the first time. The proportion varies from location to location.

Hydrothermal vents have been speculated to exist on Mars, Europa, and possibly on other icy ocean-bearing outer solar system moons. No solid evidence has been found to support or rule out these vents at this time.