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Triton is the largest moon of the planet Neptune, discovered on October 10, 1846 by William Lassell. It is the only large moon in the Solar System with a retrograde orbit, which is an orbit in the opposite direction to its planet's rotation. At 2700 km in diameter, it is the seventh-largest moon in the Solar System. Because of its retrograde orbit and composition similar to Pluto's, Triton is thought to have been captured from the Kuiper belt. Triton consists of a crust of frozen nitrogen over an icy mantle believed to cover a substantial core of rock and metal. The core makes up two-thirds of its total mass. Triton has a mean density of 2.061 g/cm³ and is composed of approximately 15–35% water ice.

Triton is one of the few moons in the Solar System known to be geologically active. As a consequence, its surface is relatively young, with a complex geological history revealed in intricate and mysterious cryovolcanic and tectonic terrains. Part of its crust is dotted with geysers believed to erupt nitrogen.

The moon was discovered by British astronomer William Lassell just 17 days after Neptune itself was discovered by German astronomers Johann Gottfried Galle and Heinrich Louis d'Arrest, who were following co-ordinates given them by French astronomer and mathematician Urbain Le Verrier.