We present a rare case of diffuse large B-cell lymphoma transformed from immunoglobulin (Ig) A-secreting marginal zone B-cell lymphoma. A 62-year-old woman was admitted to our hospital for examination of a disseminated pulmonary shadow. Gradual swelling of bilateral axilla and right inguinal lymph nodes were noted after admission. Histological examination of the lymph node biopsy specimen revealed the appearance of marginal zone B-cell lymphoma. The surface Ig of lymphoma cells was IgA-kappa, which coincided with the class of monoclonal Ig found in the patient's serum. The lymph node swelling and pulmonary shadow subsided, and the serum IgA level was normalized by 3 courses of systemic chemotherapy. However, after 4 courses of treatment, new tumor lesions at the right chest wall and left arm progressively became apparent. The biopsy specimen of the tumor showed a feature of diffuse large B-cell lymphoma. Despite intensive chemotherapy, the patient died of spreading tumor burden into the central nervous system.