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, in the Alpha Quadrant. This system was located in a border area near the Federation-Klingon border.
In 2256, this planet was mentioned with the star Tellun's name on the star chart "Alpha/Beta Quadrant Overview" in the ready room aboard the USS Discovery. (DIS: "Magic to Make the Sanest Man Go Mad")
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")