Core binding factor (CBF) leukemias, those with translocations or inversions that affect transcription factor genes RUNX1 or CBFB, account for ∼24% of adult acute myeloid leukemia (AML) and 25% of pediatric acute lymphocytic leukemia (ALL). Current treatments for CBF leukemias are associated with significant morbidity and mortality, with a 5-y survival rate of ∼50%. We hypothesize that the interaction between RUNX1 and CBFβ is critical for CBF leukemia and can be targeted for drug development. We developed high-throughput AlphaScreen and time-resolved fluorescence resonance energy transfer (TR-FRET) methods to quantify the RUNX1–CBFβ interaction and screen a library collection of 243,398 compounds. Ro5-3335, a benzodiazepine identified from the screen, was able to interact with RUNX1 and CBFβ directly, repress RUNX1/CBFB-dependent transactivation in reporter assays, and repress runx1-dependent hematopoiesis in zebrafish embryos. Ro5-3335 preferentially killed human CBF leukemia cell lines, rescued preleukemic phenotype in a RUNX1–ETO transgenic zebrafish, and reduced leukemia burden in a mouse CBFB–MYH11 leukemia model. Our data thus confirmed that RUNX1–CBFβ interaction can be targeted for leukemia treatment and we have identified a promising lead compound for this purpose.