Most pancreatic ductal adenocarcinoma (PDAC) develops from pancreatic epithelial cells bearing activating mutant KRAS genes through precancerous lesions, i.e. acinar-to-ductal metaplasia (ADM) and pancreatic intraepithelial neoplasia (PanIN). During pancreatic tumorigenesis, Hes1 expression starts with the transition from acinar cells to ADM, and continues during PanIN and PDAC formation, but the role of Hes1 in pancreatic tumorigenesis is not fully elucidated. Here we show that Hes1 plays an essential role in the initiation and progression of KRAS-driven pancreatic tumorigenesis. In vitro, activation of MAPK signaling due to EGF or mutant KRAS activation induced sustained Hes1 expression in pancreatic acinar cells. In vivo, acinar cell-specific activation of mutant KRAS by Elastase1-CreERT2;KrasG12D induced ADM/PanIN formation with Hes1 expression in mice, and genetic ablation of Hes1 in these mice dramatically suppressed PanIN formation. Gene expression analysis and lineage tracing revealed that Hes1 regulates acinar-to-ductal reprogramming-related genes and, in a Hes1-deficient state, mutant Kras-induced ADM could not progress into PanIN, but re-differentiated into acinar cells. In the Elastase1-CreERT2;KrasG12D;Trp53R172H mouse PDAC model, genetic ablation of Hes1 completely blocked PDAC formation by keeping PanIN lesions in low-grade conditions, in addition to reducing the occurrence of PanIN. Together, these findings indicate that mutant KRAS-induced Hes1 plays an essential role in PDAC initiation and progression by regulating acinar-to-ductal reprogramming-related genes.