Oncogenic NRAS and KRAS mutations are prevalent in human juvenile and chronic myelomonocytic leukemia (JMML/CMML). However, additional genetic mutations cooperating with oncogenic RAS in JMML/CMML progression and/or their transformation to acute myeloid leukemia (AML) remain largely unknown. Here we tested the potential genetic interaction of DNMT3A mutations and oncogenic RAS mutations in leukemogenesis. We found that Dnmt3a−/− induces multiple hematopoietic phenotypes after a prolonged latency, including T-cell expansion in the peripheral blood, stress erythropoiesis in the spleen and myeloid malignancies in the liver. Dnmt3a−/− significantly promoted JMML/CMML progression and shortened the survival of Kras G12D/+ mice in a cell-autonomous manner. Similarly, downregulating Dnmt3a also promoted myeloid malignancies in Nras G12D/+ mice. Further studies show that Dnmt3a deficiency rescues Kras G12D/+-mediated depletion of hematopoietic stem cells and increases self-renewal of Kras G12D/+ myeloid progenitors (MPs). Moreover,~ 33% of animals developed an AML-like disease, which is driven by Kras G12D/+; Dnmt3a−/− MPs. Consistent with our result, COSMIC database mining demonstrates that the combination of oncogenic RAS and DNMT3A mutations exclusively occurred in patients with JMML, CMML or AML. Our results suggest that DNMT3A mutations and oncogenic RAS cooperate to regulate hematopoietic stem and progenitor cells and promote myeloid malignancies.