Triton (also known as Neptune I), is Neptune's largest moon. It was discovered by British astronomer William Lassel, on October 10, 1846. This was just 17 days after Neptune itself was discovered by German astronomer Johann Gottfried Galle.

Named after the Greek sea god Triton, the son of Neptune, Triton is unique among all large moons in the solar system for its retrograde orbit around the planet.

Triton has a density of 2.05 g/cm cubed, and around 25% is water ice, the remainder being rocky material. Triton has a tenuous nitrogen atmosphere with small amounts of methane.

The surface of Triton is composed mainly of frozen nitrogen, and also has dry ice (carbon dioxide), carbon monoxide ice, water ice and methane. Large amounts of ammonia are thought to be on the surface. Triton is very bright, reflecting 60%-95% of the sunlight that reaches it. Earth's moon reflects only 11%.

Triton's total surface area is around 15.5% of the land area of Earth, or 4.5% of the total area.