Glycogen synthase kinase 3 comprises two isoforms (GSK-3α and GSK-3β) that are implicated in type II diabetes, neurodegeneration, and cancer. GSK-3 activity is elevated in human and rodent models of diabetes, and various GSK-3 inhibitors improve glucose tolerance and insulin sensitivity in rodent models of obesity and diabetes. Here, we report the generation of mice lacking GSK-3α. Unlike GSK-3β mutants, which die before birth, GSK-3α knockout (GSK-3α KO) animals are viable but display enhanced glucose and insulin sensitivity accompanied by reduced fat mass. Fasted and glucose-stimulated hepatic glycogen content was enhanced in GSK-3α KO mice, whereas muscle glycogen was unaltered. Insulin-stimulated protein kinase B (PKB/Akt) and GSK-3β phosphorylation was higher in GSK-3α KO livers compared to wild-type littermates, and IRS-1 expression was markedly increased. We conclude that GSK-3 isoforms exhibit tissue-specific physiological functions and that GSK-3α KO mice are insulin sensitive, reinforcing the potential of GSK-3 as a therapeutic target for type II diabetes.