Patients may present with a primary complaint of intra-articular knee masses causing mechanical symptoms of snapping or locking. If the history, physical examination, laboratory tests, and imaging studies point to a benign process, acceptable treatment consists of arthroscopic excision and postoperative pathology analysis. As reported in several case series, the final diagnosis can be pigmented villonodular synovitis, localized nodular synovitis, hemangioma, lipoma, or rheumatoid nodules. In this case presentation, a 39-year-old man with no previous medical conditions and a negative preoperative chest radiograph underwent arthroscopic surgery for a single intra-articular knee mass. The unexpected diagnosis, after pathology review and further medical work-up, was arthritis secondary to chronic sarcoidosis.