Waldenström macroglobulinemia (WM)/lymphoplasmacytic lymphoma (LPL) is a rare indolent B-cell neoplasm, and a gain-of-function mutation in the myeloid differentiation primary response 88 (MYD88), L265P, is a commonly recurring mutation in patients with WM/LPL. Histological transformation of WM/LPL to an aggressive lymphoma such as diffuse large B-cell lymphoma (DLBCL) is rare, and transformed DLBCL has a worse prognosis than de novo DLBCL, partly because transformed DLBCL is mostly classified as non-germinal center B-cell-like (non-GCB) subtype. We herein describe a 75-year-old man with DLBCL with a history of WM/LPL. DLBCL in this patient showed the GCB subtype, and the light chain restriction of DLBCL was different from that of the antecedent WM/LPL, indicating that the two types of lymphoma cells had distinctive origins. However, DLBCL in this patient harbored the MYD88 L265P mutation, and polymerase chain reaction and Sanger sequencing of the DLBCL and WM/LPL for immunoglobulin heavy chain gene rearrangement suggested a clonal relationship between the two lymphomas. Since the outcome of transformed DLBCL is worse than for de novo DLBCL, it is important to evaluate the clonal relationship between primary WM/LPL and the corresponding transformed DLBCL, even if the DLBCL expresses a GCB subtype or discordant light chain restriction.