Pancreatic ductal adenocarcinoma (PDAC) is a devastating disease with both an extremely poor prognosis and response to conventional chemotherapy. Identifying novel biological vulnerabilities is therefore critical in the development of more effective therapies. By using patient-derived organoids and murine genetic models, we developed a pipeline for identifying biologically active deubiquitylases (DUBs) with activity-based proteomics, coupled with a loss-of-function genetic screen in organoids. We found that ubiquitin specific protease 25 (USP25) is indispensable for PDAC progression. Compared with normal pancreatic tissue, USP25 was highly expressed and active in murine PDAC tumours and in primary human PDAC samples. Silencing of Usp25/USP25 led to reduced tumour organoid formation and viability, and transcriptional profiling revealed that USP25 is a regulator of hypoxia, glycolysis and HIF-1 signalling pathways. Mechanistically, we found that USP25 directly interacts with HIF-1 and that its deubiquitylase activity regulates HIF-1 protein stability, nuclear translocation and transcriptional activity. Moreover, treatment with a novel USP25 inhibitor resulted in a dramatic loss of murine and patient-derived organoid viability, HIF-1 signalling, and consequently induced substantial regression of murine and human organoid-derived xenografts in vivo. Thus, USP25 is a promising therapeutic target for the treatment of human PDAC.