T‐lymphoblastic leukaemia (T‐ALL) and T‐lymphoblastic lymphoma (T‐LBL) are neoplasms derived from immature lymphoid cells of T‐cell lineage. These neoplasms are biologically similar, but significant differences may exist between the two given their clinical differences. Although ample data regarding the immunophenotypic characterization T‐ALL are available, there is a paucity of such data in children and adolescents with T‐LBL. We used flow cytometry and/or immunohistochemistry to characterize the immunophenotypic profile of 180 children and adolescents with newly diagnosed T‐LBL enrolled in the Children's Oncology Group 5971 study. Multiple T‐cell, B‐cell, myeloid, and other markers were evaluated. We identified diagnostically useful immunophenotypic features of T‐LBL as well as distinct immunophenotypic subgroups, although none of these was statistically related to event‐free or overall survival in this retrospective analysis. Further studies of biologically and immunophenotypically distinct subgroups of T‐LBL, such as the early T‐cell precursor phenotype, are warranted.