MicroRNA-21 (miR-21) is one of the most abundant microRNAs in mammalian cells. It has been intensively studied for its role in regulating apoptosis and oncogenic transformation. However, the impact of miR-21 on host anti-tumor immunity remains unknown. Tumor-associated macrophages are a major leukocyte type that infiltrates tumors and predominantly develops into immunosuppressive, tumor-promoting M2-like macrophages. In contrast, the pro-inflammatory M1-like macrophages have tumoricidal activity. In this study, we show that genetic deficiency of miR-21 promotes the polarization of macrophages toward an M1-like phenotype in vivo and in vitro in the presence of tumor cells; thus it confers host mice with enhanced anti-tumor immunity. By downregulating JAK2 and STAT1, miR-21 inhibits the IFN-γ-induced STAT1 signaling pathway, which is required for macrophage M1 polarization. We also show that the expression of miR-21 in macrophages is regulated upon polarization stimuli as well as upon macrophages co-culturing with tumor cells. Thus, tumor cells may stimulate miR-21 expression in tumor-associated macrophages to prevent tumoricidal M1 polarization. However, augmented STAT1 signaling mediated by miR-21 deficiency upregulates PD-L1 expression in macrophages, which is known to inhibit phagocytic anti-tumor activity. This adverse effect can be alleviated by PD-1 blockade; indeed, miR-21 depletion in macrophages and PD-1 antibody treatment offer superior anti-tumor activity than either agent alone. These studies shed lights on potential application of the combination of miR-21 inhibition and immune checkpoint blockade to target the tumor microenvironment.