Infectious mononucleosis (IM) is a self-limiting lymphoproliferative disease usually caused by Epstein-Barr virus (EBV) infection, which predominantly infects B lymphocytes in adults. IM that occurs in T cells infected with EBV is rare and only a few cases have been reported worldwide. We describe a rare case of IM in an infant with an EBV infection predominantly in clonal cytotoxic T cells. A cervical node taken from a 10-month-old male infant was retrospectively analyzed. The architecture of the lymph node was effaced mostly with focal to extensive coagulative necrosis. In some areas, numerous infiltrating lymphocytes were detected, expressing CD2, CD3, and granzyme B, but lacked CD5 and CD56. EBV-encoded small RNAs were detected in the majority of lymphocytes. Furthermore, the T cell presented with T-cell receptor rearrangements. The patient did not do any radiation and chemotherapy. Thirty-two months follow up showed that the patient was well. We speculate IM can occur in infants through EBV infection of clonal cytotoxic T cells.