Tenosynovial giant cell tumors (TGCTs) are pigmented villonodular proliferative lesions originating from the synovium, bursa, or joint. TGCTs tend to be locally aggressive, and there is a chance for multiple occurrences, which often lead to impairment of joint function. In this article, we report the case of a diffuse-type extra-articular TGCT found in the thigh of a 36-year-old woman. Surveillance F-18 fluorodeoxyglucose positron emission tomography detected increased activity within the left thigh. This activity was confirmed with magnetic resonance imaging and with surgical excision and histopathologic determination of the tumor. This patient's case suggests that TGCTs may be discovered and followed after resection with positron emission tomography.