A case of right knee intra-articular synovial sarcoma in a 26-year-old man is reported. The patient had experienced 12 to 18 months of chronic posterior right knee pain with flexion contracture of the leg. Magnetic resonance imaging (MRI) examination demonstrated nonspecific characteristics of a well-circumscribed, homogeneous mass within the posterior lateral compartment of the knee joint. The mass was isointense and hyperintense to muscle on T1- and T2-weighted sequences respectively, and initially clinically diagnosed as a localized tenosynovial giant cell tumor. However, histological examination showed the lesion to be a synovial sarcoma arising from the synovium of the knee joint. Synovial sarcoma may have a nonspecific MR appearance, especially when less than 5 cm in size, often simulating a less aggressive process. Primary intra-articular origin may predispose to earlier presentation, and therefore, radiological evaluation of smaller masses. Nonspecific MR characteristics of small, intra-articular masses provide a diagnostic dilemma. Synovial sarcoma should be considered in the differential diagnosis when distinguishing MR characteristics of other common joint-centered entities are not present.