Tumor-associated macrophages (TAMs) have immunosuppressive capacity in mouse models of cancer. Here we show that the genetic deletion of the microRNA (miRNA)-processing enzyme DICER in TAMs broadly programs them to a CD11c+MRC1−/low M1-like immunostimulatory phenotype characterized by activated interferon-γ (IFN-γ)/STAT1/IRF signaling. M1-like TAM programming fostered the recruitment of cytotoxic T-cells (CTLs), including tumor-antigen-specific CTLs, inhibited tumor growth, and enhanced the efficacy of PD1 checkpoint blockade. Bioinformatics analysis of TAM transcriptomes identified a limited set of miRNAs putatively involved in TAM programming. Re-expression of Let-7 in Dicer-deficient TAMs was sufficient to partly rescue the M2-like (protumoral) TAM phenotype and abate tumor CTL infiltration. Targeted suppression of DICER activity in TAMs may, therefore, stimulate antitumor immunity and enhance the efficacy of cancer immunotherapy. To explore the role of DICER in the development, activation and immunological functions of TAMs, we crossed homozygous LysM-Cre (Clausen et al., 1999) with Dicerlox/lox (Harfe et al., 2005) mice to obtain mice with myeloid-cell-specific Dicer1 gene deletion (LysM-Cre;Dicer–/–, referred to as D–/–). These mice were then backcrossed to LysM-Cre to obtain the control LysM-Cre; Dicer+/+ mice (referred to as D+/+). Both LysM-Cre and Dicerlox/lox mutations were always homozygous in our experiment. We then inoculated Lewis lung carcinoma (LLC) cells subcutaneously (s.c.) in D–/– and control D+/+ mice. Once the tumors were established, we isolated by fluorescence-activated cell sorting (FACS) tumor-associated macrophages (F4/80+ cells).