Casmus III was a planet in Federation space, in the Alpha or Beta Quadrant, the third planet of the Casmus star system. By the late 23rd century, Casmus III was orbited by Starbase 42. The USS Enterprise-A traveled to the planet in 2287 on its first mission following the starship's visit to Sha Ka Ree. (TOS comics: "The Return!", "The Sentence")
- While not mentioned by name, the TOS novels: River of Blood, Seeds of Rage state that Starbase 42 orbits a planet on which dilithium had been found.