Peripheral T cell lymphomas are rare in young patients. We report the first case of a follicular variant of peripheral T cell lymphoma not otherwise specified in an 11-year-old boy, who presented with a large mediastinal mass. Microscopic examination of the mediastinal biopsy revealed nodular infiltration of medium- to large-sized atypical lymphocytes. Immunohistochemistry showed expression of follicular helper T cell markers (CD10, PD1, CXCL13, and BCL6) in tumor T cells. Epstein-Barr virus (EBV) was not detected by an in situ hybridization assay for EBV-encoded RNA. Interestingly, fluorescence in situ hybridization detected the presence in the tumor cells of the t(5;9)(q33;q22) translocation, involving ITK and SYK rearrangement. T cell clonality was detected by multiplex PCR analysis of TRG and TRD gene rearrangements. After 4 cycles of systemic chemotherapy, the patient was in complete remission. Although this entity is very rare, our observations show that lymphomas arising from T follicular helper cells may occur in children and that this should be distinguished from other lymphomas, such T-lymphoblastic lymphomas, which require a specific therapeutic approach.