Rheumatoid arthritis (RA) is a chronic autoimmune disease affecting primarily joints and an increased risk of developing malignant lymphomas in RA has been well reported. However, primary lymphoma in a joint in RA patient is rare. We report the case of a 65-year-old man with RA suffering from pain and swelling of left sternoclavicular (SC) joint, which was not relieved by adding low-dose glucocorticoid. Magnetic resonance imaging (MRI) showed a para-osseous soft tissue swelling around the SC joint and a fracture of proximal clavicle. Histology of the soft tissue demonstrated diffuse large B-cell lymphoma and the patient subsequently underwent R-CHOP (rituximab, cyclophosphamide, doxorubicin, vincristine, and prednisolone) chemotherapy. He was successfully treated with six cycles of R-CHOP chemotherapy, with discontinuation of MTX, resulting in a complete response. We performed a literature review and identified nine cases of lymphoma which involved joints in patients with rheumatoid arthritis. This is the first described case of a primary large B-cell lymphoma involving the unilateral SC joint in a patient with RA, which was initially confused with aggravation of RA. Therefore, malignant lymphoma should be considered in the differential diagnosis when a RA patient develops monoarthritis with spontaneous fracture, even without B symptoms.