A nanofibrillar cellulose hydrogel-based islet-preserving platform (iPreP) that can preserve islet viability, morphology, and function for nearly 12 weeks ex vivo, and with the ability to ameliorate glucose levels upon transplantation into diabetic hosts. Our platform has potential applications in the prolonged maintenance of human islets, providing an expanded time window for pretransplantation assessment and islet studies.