It remains a challenge to maintain human islets\u00a0ex vivo\u00a0to provide a functional and sufficient source for clinical islet transplantation and an\u00a0in vitro\u00a0model for drug screening or basic biological studies. With a conventional culture method, islets lost their functionality and mass rapidly upon they were isolated from the native environment, which provides both mechanical and biological supports. By merging the advantage of transwell cultures and the mechanical properties provided by the wood-derived nanofibrillar cellulose (NFC) hydrogel, we developed a new islet\u00a0ex vivo\u00a0platform, which can preserve human islets up to 83 days with intact morphology similar to freshly isolated counterparts. Importantly, these long-term cultured human islets are still potent and can ameliorate hyperglycemia in diabetic mice post-transplantation. The platform is designed to be\u00a0user friendly\u00a0and easily adapted to help scientists overcome the difficulties in islet cultures. Furthermore, the application of this platform aims to provide a sufficient window for pre-transplantation islet assessment, a biobanking opportunity of human islets, and an\u00a0in vitro\u00a0model for human islet studies in drug-screening, stem cell-derived islet (beta)-like organoids maturation, and basic human islet biology.