JMJD3, a stress-inducible H3K27 demethylase, plays a critical regulatory role in the initiation and progression of malignant hematopoiesis. However, how this histone modifier affects in a cell type-dependent manner remains unclear. Here, we show that in contrast to its oncogenic effect in preleukemia state and lymphoid malignancies, JMJD3 relieves the differentiation-arrest of certain subtypes (such as M2 and M3) of acute myeloid leukemia (AML) cells. RNA sequencing and ChIP−PCR analyses revealed that JMJD3 exerts anti-AML effect by directly modulating H3K4 and H3K27 methylation levels to activate the expression of a number of key myelopoietic regulatory genes. Mechanistic exploration identified a physical and functional association of JMJD3 with C/EBPβ that presides the regulatory network of JMJD3. Thus, the leukemia regulatory role of JMJD3 varies in a disease phase- and lineage-dependent manner, and acts as a potential oncorepressor in certain subsets of AML largely by coupling to C/EBPβ-centered myelopoietic program.