The Tellun system, or the Tellun star system, was an inhabited planetary system in the Alpha Quadrant. It was located near the Federation-Klingon border. The two planets of the system, Elas and Troyius, were the homeworlds of the Elasians and Troyians respectively. During the mid-23rd century, the system was considered under Federation control, but the Klingon Empire also claimed jurisdiction.
In 2268, both civilizations possessed the capability of mutual destruction. In that year, both the Federation and the Klingon Empire considered the Tellun system to reside within their respective borders. The Federation sought an end to the internal conflict and served as a mediator between Elasians and Troyians, while the Klingons tried to subvert the peace efforts in the hopes of eventually gaining access to the vast Elasian dilithium deposits.
The USS Enterprise, a Federation starship, was assigned a secret diplomatic mission to facilitate the marriage of Elaan, the Dohlman of Elas, to the ruler of Troyius. The mission was interrupted by sabotage and an indecisive ship-to-ship engagement with a Klingon D7 class battle cruiser. Regardless of the obstacles, the mission was a success. (TOS: "Elaan of Troyius"; DIS: "Magic to Make the Sanest Man Go Mad")
In 2328, this was the destination for passengers traveling from Earth Colony 2 via commercial transport. Mary Joy Macalintal, Michael Westmore, and Marion Westmore traveled there on the SS Kogin. (TNG-R: "Inheritance", okudagram)