J'Dan was a Klingon exobiologist that served in the Officer Exchange Program in the late 24th century. He was a lieutenant in the Klingon Defense Force.
In 2367, J'Dan was assigned to the Federation starship USS Enterprise-D. Since he was afflicted with Ba'ltmasor Syndrome, he had to visit sickbay often to get regular treatments. While doing this, he sometimes chatted with the medical personnel, including Doctor Crusher; the medical technician assigned to him was Crewman Simon Tarses.
Shortly after arriving on the Enterprise-D however, the dilithium articulation frame in the warp core exploded. It was suspected that it was sabotage, and Starfleet brought Rear Admiral Norah Satie out of retirement to act as Starfleet's arbitrator and to investigate what really happened. Worf discovered that J'Dan had been using a Klingon hyposyringe, which he used to treat his condition, as an optical chip reader. J'Dan used it to send schematics of the dilithium chamber of the Enterprise-D to the Romulans. He was later found innocent of blowing up the dilithium articulation frame, but sent to Qo'noS for trial for spying on the Federation for the Romulans. (TNG: "The Drumhead")