Most primary ocular adnexal lymphomas are extranodal marginal zone B-cell lymphomas of mucosa-associated lymphoid tissue (MALT). A few cases of ocular adnexal mantle cell lymphomas have been reported in the literature. We present a case of mantle cell lymphoma presenting as conjunctival mass. A 58-year-old man presented with a palpable mass in the left lower tarsal conjunctiva incidentally detected one month previously. Histopathologic examination showed proliferation of monomorphous small-to-medium sized lymphoid cells. On immunohistochemistry, tumor cells were positive for CD20, bcl-2, and cyclin D1, and negative for CD5. PCR analysis for immunoglobulin heavy chain gene rearrangement showed monoclonal B-cell proliferation. t(11;14)(q13;q32), involving the CCND1 and IGH genes, was detected in interphase fluorescent in situ hybridization using formalin-fixed, paraffin-embedded tissue; however, MALT1 gene translocation was not observed. The final diagnosis was mantle cell lymphoma. There was no lymphadenopathy; however, bone marrow involvement of the lymphoma was suspected. The patient has been receiving systemic chemotherapy. This case emphasizes the differential diagnosis of conjunctival mantle cell lymphoma from extranodal marginal zone B-cell lymphomas of MALT regarding the clinical and pathological aspects.