Diabetes is highly prevalent in individuals with pancreatic ductal adenocarcinoma (PDAC) and even precedes diagnosis of PDAC; however, the mechanisms of pancreatic cancer-associated blood glucose deterioration remain largely unknown. Here, we constructed a prospective cohort of patients undergoing pancreatectomy to investigate the underlying mechanism of PDAC-associated hyperglycemia. A total of 160 patients who underwent pancreatectomy (72 patients with PDAC and 88 patients without PDAC) were enrolled at a tertiary care hospital. Glucometabolic parameters under oral glucose tolerance test were assessed in both pre- and postoperative periods, and patient-derived blood and pancreatic tissue samples were collected. Compared with patients without PDAC, patients with PDAC showed severe hyperglycemia with impaired insulin secretion before surgery. However, despite identical type of pancreatectomy in both groups, hyperglycemia improved more significantly and insulin secretory function declined less after pancreatectomy in patients with PDAC. Plasma Wnt5a and pancreatic islet β-catenin levels were higher in patients with PDAC and correlated with the degree of hyperglycemia and insulin deficiency. Plasma Wnt5a levels also correlated with tumor size and pancreatic islet β-catenin expression in patients with PDAC. In rodent islets, Wnt5a treatment suppressed insulin release, which was recovered by inhibition of β-catenin. Collectively, impaired pancreatic insulin secretion by aberrant Wnt5a/β-catenin activation may underlie the hyperglycemia associated with PDAC. Our finding provides insights into the unique molecular mechanism of pancreatic cancer-associated hyperglycemia, paving the way for the identification of potential biomarker and therapeutic targets for this condition.