Diabetes is now generally accepted as a crucial event in the process of pancreatic cancer (PaC). However, molecular mechanisms underlying the relationship between diabetes and PaC are not fully understood. Regenerating gene (REG) Iα is a growth factor affecting pancreatic islet beta cells, and it has been shown to be involved in the carcinogenesis in gastrointestinal tract. It is rational to speculate that REG Iα plays a potential role in the pathogenesis of PaC with diabetes. The aim of this study was to evaluate the REG Iα protein expression profile in PaC with and without diabetes, and define the contribution of REG Iα on PaC development. We found that REG Iα protein preferentially expressed in cancerous tissues of PaC patients with diabetes by Western blot. REG Iα positive cancer cells in PaC with diabetes (n = 38) was significantly higher than that in subjects without diabetes (n = 42, p < 0.05) by immunohistochemical analysis. Furthermore, we found that overexpression of REG Iα protein in PaC cell lines resulted in accelerated cell proliferation and consequently tumor growth, both in vitro and in vivo. The findings suggest that REG Iα may act as one of the tumor promoter and contribute to the aggressive nature of PaC, especially in the subpopulation with diabetes. This study would shed new insights for understanding the molecular mechanisms underlying the link between diabetes and PaC.