Giant cell tumor of the tendon sheath (GCTTS) occurs most often in the hand and rarely in the feet, and as an extremely uncommon presentation in the knee joint. Case reports involving GCTTS in the knee joint generally describe it originating from the nearby anterior cruciate ligament, posterior cruciate ligament, patellar tendon, and medial plica. To the best of our knowledge, there are no previously reported case reports involving GCTTS originating in the ligamentum mucosum. In this article, we describe a 27-year-old male patient who was admitted to the orthopedic emergency room with a painful locked knee. He had severe pain that was worse with activity and a decreased range of motion. Magnetic resonance imaging (MRI) indicated massive swelling and a well-circumscribed lobulated intraarticular mass at the distal one third of the ligamentum mucosum. The mass was removed successfully with arthroscopic-assisted mini-open excision, and histological analysis subsequently diagnosed it as a localized type of GCSTT. The patient remained asymptomatic and a follow-up MRI two years after surgery did not show any recurrence of the lesion.