High macrophage infiltration in cancer is associated with reduced survival in animal models and in patients. This reflects a shift in the macrophage response from a tumor-suppressive to tumor-supportive program governed by transcriptional events regulated by the inflammatory milieu. Although several transcription factors are known to drive a prometastatic program, those that govern an antimetastatic program are less understood. IFN regulatory factor-8 (IRF8) is integral for macrophage responses against infections. Using a genetic loss-of-function approach, we tested the hypothesis that IRF8 expression in macrophages governs their capacity to inhibit metastasis. We found that:(a) metastasis was significantly increased in mice with IRF8-deficient macrophages;(b) IRF8-deficient macrophages displayed a program enriched for genes associated with metastasis; and (c) lower IRF8 expression correlated with reduced survival in human breast and lung cancer, as well as melanoma, with high or low macrophage infiltration. Thus, a macrophage hi IRF8 hi signature was more favorable than a macrophage hi IRF8 lo signature. The same held true for a macrophage lo IRF8 hi vs. a macrophage lo IRF8 lo signature. These data suggest that incorporating IRF8 expression levels within a broader macrophage signature or profile strengthens prognostic merit. Overall, to our knowledge, our findings reveal a previously unrecognized role for IRF8 in macrophage biology to control metastasis or predict outcome.