Immune suppression in the tumor microenvironment (TME) is a central obstacle to effective immunotherapy. Tumor-associated macrophages (TAMs) are key components of the TME. Although TAMs have been viewed as an ideal target of intervention to steer immunity in cancer treatment, the approach has been hampered by the lack of knowledge of how TAM plasticity is controlled by cell intrinsic factors. VentX is a homeobox protein implicated in proliferation and differentiation of human hematopoietic and immune cells. Using clinical samples obtained from cancer patients, we find that VentX expression is drastically reduced in TAMs. We show here that VentX promotes M1 differentiation of TAMs, and that VentX-regulated TAMs, in turn, revert immune suppression at the TME. Using a NSG mouse model of human colon cancers, we demonstrate that VentX regulates TAM function in tumorigenesis in vivo. Our findings suggest a mechanism underlying immune suppression at TME and potential applications of VentX-regulated TAMs in cancer immunotherapy.