Significance Here we report that targeting the pattern recognition receptor MARCO on macrophages within the tumor alters their polarization and in turn activate natural killer (NK) cells to kill the tumor. We describe the mechanism of action, including that NK cells are induced to use receptor-mediated killing of the cancer cells. Furthermore, we find that this type of treatment works in combination with T cell-targeted checkpoint therapies. We then transfer this finding to humans and find that similar subpopulations of human macrophages also block NK cells. Finally, we find that targeting these human macrophages with a new specific antibody that we generated can activate these to release NK cell killing, thus generating a venue for combinatory treatments for cancer.