Regulation of insulin secretion along the secretory pathway is incompletely understood. We addressed the expression of SIL1, a nucleotide exchange factor for the endoplasmic reticulum (ER) chaperone glucose-regulated protein 78 kD (GRP78), in pancreatic beta cells and investigated whether or not SIL1 is involved in beta cell function.SIL1 expression was analysed by immunoblotting and immunofluorescence. Metabolic and islet variables, including glucose tolerance, beta cell mass, insulin secretion, islet ultrastructure, insulin content and levels of ER stress marker proteins, were addressed in Sil1 knockout (Sil1 (-/-)) mice. Insulin, proinsulin and C-peptide release was addressed in Sil1 (-/-) islets, and SIL1 overexpression or knockdown was explored in MIN6 cells in vitro. Models of type 1 diabetes and insulin resistance were induced in Sil1 (-/-) mice by administration of streptozotocin (STZ) and a high-fat diet (HFD), respectively.We show that SIL1 is expressed in pancreatic beta cells and is required for islet insulin content, islet sizing, glucose tolerance and glucose-stimulated insulin secretion in vivo. Levels of pancreatic ER stress markers are increased in Sil1 (-/-) mice, and Sil1 (-/-) beta cell ER is ultrastructurally compromised. Isolated Sil1 (-/-) islets show lower proinsulin and insulin content and impaired glucose-stimulated insulin secretion. Modulation of SIL1 protein levels in MIN6 cells correlates with changes in insulin content and secreted insulin. Furthermore, Sil1 (-/-) mice are more susceptible to STZ-induced type 1 diabetes with increased apoptosis. Upon HFD feeding, Sil1 (-/-) mice show markedly lower insulin secretion and exacerbated glucose intolerance compared with control mice. Surprisingly, however, HFD-fed Sil1 (-/-) mice display pronounced islet hyperplasia with low amounts of insulin in total pancreas.These results reveal a novel role for the nucleotide exchange factor SIL1 in pancreatic beta cell function under physiological and disease conditions such as diabetes and the metabolic syndrome.