The genes encoding RAS family members are frequently mutated in juvenile myelomonocytic leukemia (JMML) and acute myeloid leukemia (AML). RAS proteins are difficult to target pharmacologically; therefore, targeting the downstream PI3K and RAF/MEK/ERK pathways represents a promising approach to treat RAS-addicted tumors. The p110α isoform of PI3K (encoded by Pik3ca) is an essential effector of oncogenic KRAS in murine lung tumors, but it is unknown whether p110α contributes to leukemia. To specifically examine the role of p110α in murine hematopoiesis and in leukemia, we conditionally deleted p110α in HSCs using the Cre-loxP system. Postnatal deletion of p110α resulted in mild anemia without affecting HSC self-renewal; however, deletion of p110α in mice with KRASG12D-associated JMML markedly delayed their death. Furthermore, the p110α-selective inhibitor BYL719 inhibited growth factor–independent KRASG12D BM colony formation and sensitized cells to a low dose of the MEK inhibitor MEK162. Furthermore, combined inhibition of p110α and MEK effectively reduced proliferation of RAS-mutated AML cell lines and disease in an AML murine xenograft model. Together, our data indicate that RAS-mutated myeloid leukemias are dependent on the PI3K isoform p110α, and combined pharmacologic inhibition of p110α and MEK could be an effective therapeutic strategy for JMML and AML.