UNLABELLED: The gold standard for assessing the function of human islets or β-like cells derived from stem cells involves their engraftment under the kidney capsule of hyperglycemic, immunodeficient mice. Current models, such as streptozotocin treatment of severely immunodeficient mice or the NRG-Akita strain, are limited due to unstable and variable hyperglycemia and/or high morbidity. To address these limitations, we developed the IsletTester mouse via CRISPR/Cas9-mediated gene editing of glucokinase (Gck), the glucose sensor of the β-cells, directly in NSG zygotes. IsletTester mice are heterozygous for an Arg345→stop mutation in Gck and present with stable random hyperglycemia (∼250 mg/dL [14 mmol/L]), normal lifespan, and fertility. We demonstrate the utility of this model through functional engraftment of both human islets and human embryonic stem cell-derived β-like cells. The IsletTester mouse will enable the study of human islet biology over time and under different physiological conditions and can provide a useful preclinical platform to determine the functionality of stem cell-derived islet products. ARTICLE HIGHLIGHTS: Current mouse models for assessing islet function in vivo are limited due to unstable and variable hyperglycemia and/or high morbidity. We derived the IsletTester mouse to address these limitations. Leveraging a previously characterized glucokinase mutation and CRISPR/Cas9 technology, we successfully developed a moderately hyperglycemic and immunodeficient mouse model for the in vivo assessment of islet function. Our IsletTester mouse has stable, moderate hyperglycemia that can be corrected with primary human islets or stem cell-derived insulin-producing cells. The IsletTester mouse provides a reliable, easy-to-use platform for the preclinical assessment of stem cell-derived islet products or islet-targeted drugs.