The molecular mechanisms leading to resistance to PD-1 blockade are largely unknown. Here, we characterize tumor biopsies from a patient with melanoma who displayed heterogeneous responses to anti-PD-1 therapy. We observe that a resistant tumor exhibited a loss-of-function mutation in the tumor suppressor gene , whereas a sensitive tumor from the same patient did not. Consistent with a functional role in immunotherapy response, inactivation of in murine tumor cell lines caused resistance to anti-PD-1 in immunocompetent animals. Loss of was associated with altered immune microenvironment, decreased tumor-intrinsic expression of the double-stranded RNA (dsRNA) sensors MDA5 and RIG1, and diminished induction of type I IFN and MHC-I expression. In contrast, restoration of dsRNA sensing in -deficient cells was sufficient to sensitize them to anti-PD-1. Our results thus establish a new role for the commonly inactivated tumor suppressor in viral sensing and sensitivity to immunotherapy. SIGNIFICANCE: Our findings establish a role of the commonly inactivated tumor suppressor as a genomic driver of response to anti-PD-1 therapy. loss promotes resistance to anti-PD-1 through the downregulation of viral sensing pathways, suggesting that therapeutic reactivation of these pathways could improve clinical responses to checkpoint inhibitors in genomically defined cancer patient populations..