Immune checkpoint blockade (ICB) leads to durable and complete tumour regression in some patients but in others gives temporary, partial or no response. Accordingly, significant efforts are underway to identify tumour-intrinsic mechanisms underlying ICB resistance. Results from a published mouse model CRISPR screen suggested that targeting an E3 ligase (STUB1) involved with protein homeostasis, may overcome ICB resistance but the molecular basis behind this observation is unclear. Using the ICB-resistant and poorly immunogenic B16-F10 murine melanoma model, we reveal an under-appreciated role of STUB1 to dampen the interferon gamma (IFNγ) response. Deletion of Stub1 in tumour cells increased IFNGR1 abundance on cellular surface, thus lowering the stimulating threshold of IFNγ. These outcomes translated to IFNγ-enhanced antigen presentation and upregulation of the immunoproteasome complexes. Through proteomics and gene expression profiling, we confirmed STUB1 as a negative regulator of the IFNγ signaling pathway. To block the function of STUB1 in tumour cells, we stably expressed a rationally designed inhibitory biologic, which recapitulated the Stub1-null phenotypes in both murine and human tumour cells. Overall, our findings elucidate STUB1 as a barrier for IFNγ sensing and offer a roadmap to pursue STUB1 inhibitors, which may improve tumour response to checkpoint inhibitory therapy.