Reduced pancreatic β-cell function or mass is the critical problem in developing diabetes. Insulin release from β-cells depends on Ca²⁺ influx through high voltage–gated Ca²⁺ channels (HVCCs). Ca²⁺ influx also regulates insulin synthesis and insulin granule priming and contributes to β-cell electrical activity. The HVCCs are multisubunit protein complexes composed of a pore-forming α₁ and auxiliary β and α₂δ subunits. α₂δ is a key regulator of membrane incorporation and function of HVCCs. Here we show that genetic deletion of α₂δ-1, the dominant α₂δ subunit in pancreatic islets, results in glucose intolerance and diabetes without affecting insulin sensitivity. Lack of the α₂δ-1 subunit reduces the Ca²⁺ currents through all HVCC isoforms expressed in β-cells equally in male and female mice. The reduced Ca²⁺ influx alters the kinetics and amplitude of the global Ca²⁺ response to glucose in pancreatic islets and significantly reduces insulin release in both sexes. The progression of diabetes in males is aggravated by a selective loss of β-cell mass, while a stronger basal insulin release alleviates the diabetes symptoms in most α₂δ-1^−/− female mice. Together, these findings demonstrate that the loss of the Ca²⁺ channel α₂δ-1 subunit function increases the susceptibility for developing diabetes in a sex-dependent manner.