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One of a pair of outer satellites of the planet Neptune. With its larger companion moon Neso, Psamathe follows a retrograde orbit in the distant depths of the Neptunian system. Indeed, Psamathe and Neso can travel farther from Neptune than Mercury's distance from the Sun, and Neso holds the record for the distance of a moon from its primary planet (with Psamathe a close second). This great distance places Psamathe and Neso at the limit for a sustainable orbit around Neptune.
Similarities in the orbital paths of Psamathe and Neso imply that they had a shared origin, probably in the break-up of a larger moon, or an object captured by Neptune's gravity. Psamathe's highly inclined and eccentric orbit takes it just under twenty-five years to complete. The moon itself is about forty kilometres in diameter (about two thirds of the diameter of the outer moon Neso).
Following convention, Psamathe takes its name from classical figure associated with the sea (due to its association with Neptune, named for the Roman god of the sea). In Psamathe's case (and also that of Neso) the name comes from among the fifty Nereids, the daughters of Nereus the Old Man of the Sea.
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