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Introduction: Giant cell tumor of the tendon sheath (GCTTS) is a benign tumor which most commonly affects hand tendons but rarely affects the large joints. GCT-TS usually occurs in individuals between the ages of 30 and 50 years, with a predominance for females, and exhibits the tendency for local recurrence following surgical resection. Marginal excision is the standard treatment of GCT-TS.
Case Presentation: The case studied is a 51-year-old female with a one-year history of gradual onset of pain in the left knee. She denied any history of trauma. The pain was worse at night and during walking and she was on regular pain medication. She was referred as a case of osteoarthritis not responding to conservative treatment. The purpose of this report is to emphasize the possibility of other pathologies whenever the pain is not responding to usual conservative treatment even in the presence of radiological features.
Discussion: GCT-TS in large joints may be more difficult to diagnose, as there are few and nonspecific symptoms. GCTTS most frequently occurs in the digits of the hands and feet and is rarely seen in the larger joints. Although GCT-TS is a benign tumor, it has a high incidence of recurrence following resection (10-40%).
Conclusion: Giant cell tumor of the suprapatellar region is unusual especially when accompanied by pain presentation in a patient who is diagnosed as another case of osteoarthritis. Careful surgical excision is effective in preventing recurrence despite relatively high recurrence rates.

Keywords: Knee joint; Giant cell tumor; Arthroscopic excision; Osteoarthritis