Activating KRAS mutations are detected in a substantial number of hematologic malignancies. In a murine T-cell acute lymphoblastic leukemia (T-ALL) model, we previously showed that expression of oncogenic Kras induced a premalignant state accompanied with an arrest in T-cell differentiation and acquisition of somatic Notch1 mutations. These findings prompted us to investigate whether the expression of oncogenic KRAS directly affects DNA damage repair. Applying divergent, but complementary, genetic approaches, we demonstrate that the expression of KRAS mutants is associated with increased expression of DNA ligase 3α, poly(ADP-ribose) polymerase 1 (PARP1), and X-ray repair cross-complementing protein 1 (XRCC1), all essential components of the error-prone, alternative nonhomologous end-joining (alt-NHEJ) pathway. Functional studies revealed delayed repair kinetics, increased misrepair of DNA double-strand breaks, and the preferential use of microhomologous DNA sequences for end joining. Similar effects were observed in primary murine T-ALL blasts. We further show that KRAS-mutated cells, but not KRAS wild-type cells, rely on the alt-NHEJ repair pathway on genotoxic stress. RNA interference-mediated knockdown of DNA ligase 3α abolished resistance to apoptotic cell death in KRAS-mutated cells. Our data indicate that targeting components of the alt-NHEJ pathway sensitizes KRAS-mutated leukemic cells to standard chemotherapeutics and represents a promising approach for inducing synthetic lethal vulnerability in cells harboring otherwise nondruggable KRAS mutations.