We investigated the consequence of reducing GRP78 abundance in pancreatic β-cells of adult mice in normal physiological conditions. Genetic Grp78 reduction caused β-cell failure and diabetes in male mice, with weight loss, hyperglycemia, glucose intolerance, β-cell mass reduction, and β-cell dedifferentiation. Male and female mice showed similar increases in β-cell death, but proliferation was more profoundly increased in females. Ex vivo Grp78 deletion led to hyperactivation of ATF6 target genes in male islet cells compared with female. Preserving β-cell GRP78 abundance may reduce the likelihood of diabetes, especially in males.