The treatment for giant cell tumors of the tendon sheath is surgical therapy, but surgical recurrence rates were reported to be as high as 50% in some cases. Therefore, complete radical excision of the lesion is the treatment of choice. If the tumor originates from the joint, it is important to perform capsulotomy. Here, the authors report the first case of successful treatment of a localized intra-articular giant cell tumor in the wrist by arthroscopic resection.A 28-year-old right-handed woman visited the clinic because of left wrist ulnar-side pain, which had been aggravated during the previous 15 days. Vague ulnar-side wrist pain had begun 2 years ago. When the authors examined the patient, the wrist showed mild swelling on the volo-ulnar aspect and the distal radioulnar joint, as well as volar joint line tenderness. She showed a positive result on the ulnocarpal stress test and displayed limited range of motion. Magnetic resonance imaging revealed an intra-articular mass with synovitis in the ulnocarpal joint. Wrist arthroscopy was performed using standard portals under regional anesthesia. The arthroscopic findings revealed a large, well-encapsulated, yellow lobulated soft-tissue mass that was attached to the volar side of the ulnocarpal ligament and connected to the extra-articular side. The mass was completely excised piece by piece with a grasping forceps. Histopathologic examination revealed that the lesion was an intra-articular localized form of a tenosynovial giant cell tumor.At 24-month follow-up, the patient was completely asymptomatic and had full range of motion in her left wrist, and no recurrence was found in magnetic resonance imaging follow-up evaluations.The authors suggest that the arthroscopic excision of intra-articular giant cell tumors, as in this case, may be an alternative method to open excisions, with many advantages.