Marginal zone lymphoma (MZL) is a low-grade B-cell lymphoma derived from marginal-zone B cells. Because of a lack of specific immunohistochemical markers, MZL is mainly diagnosed based on the cytological appearance and growth pattern of the tumor. Marginal-zone B cells were recently shown to selectively express immunoglobulin superfamily receptor translocation-associated 1 (IRTA1), but the antibody used in that study is not commercially available. We therefore investigated the IRTA1 expression in non-neoplastic lymphoid tissues and 261 malignant lymphomas, examining the ability of a commercially available antibody to accurately diagnose MZL. Among 29 MZLs, 23 of 25 extranodal MZL of mucosa-associated lymphoid tissue (MALT lymphomas), 3 of 6 splenic MZLs and 3 of 6 nodal MZLs were positive for IRTA1. Among the 98 diffuse large B-cell lymphomas (DLBCLs), 33 were positive for IRTA1, including 1 of 38 follicular lymphomas (FLs), and all precursor B-lymphoblastic (2 of 2) and T-lymphoblastic (7 of 7) leukemia/lymphomas. Other mature B-cell and T-cell lymphomas, and Hodgkin lymphoma were negative for IRTA1. In MALT lymphoma, positive cells were detected mainly in intraepithelial and subepithelial marginal-zone B cells. In one case of grade 3 FL, IRTA1 was also expressed in the area of large-cell transformation. When tumors were classified as germinal-center B cell–like (GCB) or non-GCB using the algorithm of Hans, positive expression of IRTA1 was correlated significantly with non-GCB DLBCLs (P < .05). These results demonstrated the ability of the commercially available IRTA1 antibody to distinguish MALT lymphoma from other low-grade B-cell lymphomas.