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 Oncocytic thyroid carcinoma (OTC) and tenosynovial giant cell tumor (TGCT) are rare neoplasms with distinct biological behavior, and their coexistence has not been previously well documented. We report a rare case of concurrent recurrent OTC and diffuse-type TGCT (TGCT-D) in a 45-year-old man who presented with a rapidly enlarging soft-tissue mass involving the left ankle and foot. ¹⁸F-fluorodeoxyglucose positron emission tomography/computed tomography (FDG PET/CT) demonstrated intensely FDG-avid lesions in the thyroid bed consistent with recurrent disease, as well as a hypermetabolic infiltrative musculoskeletal lesion in the left ankle and foot, initially suspected to represent metastatic spread. However, histopathological examination of the ankle lesion established the diagnosis of TGCT-D. This case illustrates an important diagnostic pitfall in oncology imaging: FDG-avid musculoskeletal lesions in patients with known malignancy do not necessarily represent metastases. Careful integration of clinical information, cross-sectional imaging, particularly MRI, and histopathological confirmation is essential to avoid misdiagnosis and to ensure appropriate management.