Psamathe ( /ˈsæməθiː/ SAM-ə-thee; Latin: Psamathē; Greek: Ψαμάθη), also known as Neptune X, is a retrograde irregular satellite of Neptune. It is named after Psamathe, one of the Nereids. This moon was discovered by Scott S. Sheppard and David C. Jewitt in 2003 using the 8.2 meter Subaru telescope. Before the announcement of its name on February 3, 2007 (IAUC 8802), it was known by the provisional designation S/2003 N 1.
This moon is about 38 kilometers in diameter. It orbits Neptune at a distance of about 46,695,000 km and requires almost 25 Earth years to make one orbit. The orbit of this satellite is close to the theoretical stable separation from Neptune for a body in a retrograde orbit. Given the similarity of this moon's orbital parameters with Neso (S/2002 N 4), it was suggested that both irregular satellites could have a common origin in the break-up of a larger moon. Both are further from their primary than any other known moon in the Solar System.