Troyius was an inhabited planet and the homeworld of the Troyians, a spacefaring humanoid species. The planet was farther from its primary than its sister planet Elas. It was located in the Tellun system. This system was located in a border area near the Federation-Klingon border.
The planet had rich deposits of dilithium ("radan" in the Troyian language). In 2268, both planets possessed the capability of mutual destruction. The wedding of the Dohlman of Elas, Elaan, to the ruler of Troyius preserved the peace. (TOS: "Elaan of Troyius")