This case report details a previously undescribed malignancy of the tendon sheath in a golden retriever. This dog originally presented with lameness of the left forelimb, at which point radiographs revealed a monostotic, lytic lesion of the distal radius with overlying soft-tissue swelling. A fine-needle aspirate was performed, and cytology was compatible with a sarcoma, with the primary differential being an osteosarcoma. After amputation, the leg was submitted for histopathology, which revealed inconsistencies with a typical osteosarcoma lesion, including lack of osteoid deposition. Second opinion histopathology showed a fibrosarcoma that appeared to have originated in the tendon sheath of an extensor tendon and then secondarily invaded the radius. At the time of publication, ∼17 mo after amputation, the dog continues to do well without any evidence of recurrent or metastatic disease.