B-acute lymphoblastic leukemia (B-ALL) represents the most common pediatric hematological tumor that derives from the aberrant proliferation of early B lymphocytes in the bone marrow. Although most of the B-ALL children take advantage from current therapeutic protocols, some patients relapse and need alternative therapies. With this background, we investigated whether interleukin (IL)-27, an immunomodulatory cytokine with antitumor properties, may function as an antitumor agent against pediatric B-ALL cells. Here we show for the first time that pediatric B-ALL cells functional IL-27R and that IL-27 dampens directly tumor growth in vivo and in vitro through mechanisms elucidated in this study. The novelty of these results deals with the first demonstration that (1) B-ALL cells from pediatric patients injected intravenously (i.v.) into NOD/SCID/Il2rg(-/-) (NSG) mice gave rise to leukemic spreading that was severely hampered by IL-27; (2) IL-27-treated mice, compared with controls, showed significant reduction of putative B-ALL-initiating cells and blasts in the peripheral blood (PB), bone marrow (BM) and spleen; and that (3) IL-27 reduced in vitro B-ALL cell proliferation and angiogenesis, induced apoptosis and downregulated miR-155. Our results strongly encourage the development of future clinical trials to evaluate the toxicity and efficacy of IL-27 in childhood B-ALL patients.