Primary central nervous system T-cell lymphoma (PCNSTCL) is rare, accounting for 2% of CNS lymphomas. We report the first case of PCNSTCL with aberrant expression of CD20 and CD79a in an 81-year-old man with a left periventricular brain mass. A biopsy revealed dense lymphoid infiltrate consisting of medium-sized cells in a background of gliosis and many histiocytes. The lymphoid cells were positive for CD2, CD3, CD7, CD8, T-cell intracellular antigen-1, granzyme B, CD20, and CD79a and negative for CD4, CD5, PAX-5, OCT-2, BOB-1, human herpes virus-8, and Epstein-Barr virus–encoded small RNAs. Molecular studies revealed clonal TCR-β and TCR-γ gene rearrangements and negative immunoglobulin gene rearrangements. The patient was treated with chemotherapy (vincristine and methotrexate) and rituximab, but he died 1 month after the diagnosis. This is a unique case that emphasizes the use of a multimodal approach, including a broad immunohistochemical panel and molecular studies in lineage determination for lymphomas with ambiguous phenotype.