Primary central nervous system (CNS) lymphomas are relatively rare with the most common subtype being diffuse large B-cell lymphoma. Primary CNS T-cell lymphomas (PCNSTL) account for <5% of CNS lymphomas. We report the clinical, morphologic, immunophenotypic, and molecular characteristics of 18 PCNSTLs. Fifteen cases were classified as peripheral T-cell lymphoma, not otherwise specified, 2 of which were of γδ T-cell derivation and 1 was TCR silent; there was 1 anaplastic large cell lymphoma, ALK-positive and 2 anaplastic large cell lymphoma, ALK-negative. Median age was 58.5 years (range, 21 to 81 y), with an M:F ratio of 11:7. Imaging results showed that 15 patients had supratentorial lesions. Regardless of subtype, necrosis and perivascular cuffing of tumor cells were frequently observed (11/18 cases). CD3 was positive in all cases but 1; 10/17 were CD8-positive, and 5/17 were CD4-positive. Most cases studied had a cytotoxic phenotype with expression of TIA1 (13/15) and granzyme-B (9/13). Polymerase chain reaction analysis of T-cell receptor γ rearrangement confirmed a T-cell clone in 14 cases with adequate DNA quality. Next-generation sequencing showed somatic mutations in 36% of cases studied; 2 had >1 mutation, and none showed overlapping mutations. These included mutations in DNMT3A, KRAS, JAK3, STAT3, STAT5B, GNB1, and TET2 genes, genes implicated previously in other T-cell neoplasms. The outcome was heterogenous; 2 patients are alive without disease, 4 are alive with disease, and 6 died of disease. In conclusion, PCNSTLs are histologically and genomically heterogenous with frequent phenotypic aberrancy and a cytotoxic phenotype in most cases.