Polyarticular joint manifestations as the predominant symptom of non-Hodgkin's lymphoma (NHL) are quite rare. In the absence of peripheral lymph node and visceral involvement, lymphomas presenting as polyarthritis create a problem for the patients as another rheumatic disease. We present a case that had been diagnosed with rheumatoid arthritis because of symmetrical articular symptoms. The patient later developed severe pain and marked swelling in her right fourth finger, and a diagnosis of septic arthritis and osteomyelitis complicating rheumatoid arthritis was assumed. The final diagnosis of NHL with synovial involvement could be made only after histopathologic examination of a biopsy specimen obtained from the amputated finger. This is the first case report demonstrating direct synovial involvement of a small joint in a patient with NHL presenting with polyarthritis. Articular and periarticular involvement of multiple joints shown by MRI in this patient suggests that direct synovial involvement may be responsible for the polyarticular symptoms in such patients.