DNA methyltransferase 3A (DNMT3A) is frequently mutated in hematological cancers; however, the underlying oncogenic mechanism remains elusive. Here, we report that the DNMT3A mutational hotspot at Arg882 (DNMT3A^R882H) cooperates with NRAS mutation to transform hematopoietic stem/progenitor cells and induce acute leukemia development. Mechanistically, DNMT3A^R882H directly binds to and potentiates transactivation of stemness genes critical for leukemogenicity including Meis1, Mn1, and Hoxa gene cluster. DNMT3A^R882H induces focal epigenetic alterations, including CpG hypomethylation and concurrent gain of active histone modifications, at cis-regulatory elements such as enhancers to facilitate gene transcription. CRISPR/Cas9-mediated ablation of a putative Meis1 enhancer carrying DNMT3A^R882H-induced DNA hypomethylation impairs Meis1 expression. Importantly, DNMT3A^R882H-induced gene-expression programs can be repressed through Dot1l inhibition, providing an attractive therapeutic strategy for DNMT3A-mutated leukemias.