The protein tyrosine phosphatase CD45, encoded by the PTPRC gene, is well known as a regulator of B- and T-cell receptor signaling. In addition, CD45 negatively regulates JAK family kinases downstream of cytokine receptors. Here, we report the presence of CD45 inactivating mutations in T-cell acute lymphoblastic leukemia. Loss-of-function mutations of CD45 were detected in combination with activating mutations in IL-7R, JAK1, or LCK, and down-regulation of CD45 expression caused increased signaling downstream of these oncoproteins. Furthermore, we demonstrate that down-regulation of CD45 expression sensitizes T cells to cytokine stimulation, as observed by increased JAK/STAT signaling, whereas overexpression of CD45 decreases cytokine-induced signaling. Taken together, our data identify a tumor suppressor role for CD45 in T-cell acute lymphoblastic leukemia.