Tenosynovial giant cell tumors (TGCT) are a group of generally benign intra-articular and soft tissue tumors with common histological features. TGCT is also known as pigmented villonodular synovitis. There are localized and diffuse forms. Localized types include giant cell tumors of tendon sheath and localized pigmented villonodular synovitis, whereas diffuse types encompass conventional pigmented villonodular synovitis and diffuse type giant cell tumor. Localized tumors are generally indolent, whereas diffuse tumors are locally aggressive. In this article, we report the case of a diffuse-type extra-articular TGCT found in the left thigh of a 73-year-old woman who presented with a painless but gradually progressive swelling in the left thigh since eighteen months. On examination, there was a soft cystic swelling measuring 22 cm. The swelling was fixed to the underlying soft tissues. She had a Computed Tomography scan of the left thigh showing a mass of fluid density, well encapsulated between the muscles of the thigh, measuring 20x10 cm of major axes, compressing the femoral vessels without invading them. The patient had a complete marginal resection of the tumor. The immune-histopathological findings were consistent with those of a diffuse type of Giant Cell Tenosynovial Tumor. At 18 months follow-up, the patient is asymptomatic with no evidence of disease recurrence. The extra-articular diffuse type TGCT is more aggressive than the localized type. Although these tumors are benign in the majority of cases, malignant transformation has been reported. Therefore, close follow-up is recommended after tumor excision.