Islet transplantation is a promising therapy for insulin-dependent diabetes. However, immune rejection and insufficient vascularization hinder the survival and function of transplanted islets. Here, we show effective engraftment of vascularized and functional mouse and rat islets transplanted into biomaterial-remodeled spleens of nonimmunosuppressed rodents and human islets transplanted into the remodeled spleens of nonhuman primates (NHPs) on varying degrees of immunosuppression. We found evidence that konjac glucomannan-modified silica nanoparticles (KSiNPs) remodeled the spleen into an extracellular matrix (ECM)-rich, immunosuppressive niche to support the survival of syngeneic or xenogeneic islets. Transplanted islets in the remodeled spleens showed improved engraftment, neovascularization, and functionality and restored normoglycemia in streptozotocin (STZ)-induced type 1 diabetic models in the mice and macaques, with stable insulin and C-peptide secretion in mice for 90 days and macaques for 28 days. KSiNP injection and islet transplantation into macaque spleens under B-ultrasound guidance were preclinically feasible. These findings highlight the safety and effectiveness of spleen tissue remodeling in supporting the survival and function of transplanted islets, providing a promising strategy for treating type 1 diabetes mellitus (T1DM).