Macroencapsulation systems for pancreatic islet delivery are an emerging strategy for cell replacement therapy in type 1 diabetes, aimed at eliminating systemic immunosuppression and enabling the use of alternative cell sources. A critical design challenge is to achieve high packing densities while maintaining sufficient oxygen and nutrient supply to ensure islet viability and function. This design-focused study presents a one-step method for the simultaneous and defined formation and spatial arrangement of uniformly sized pancreatic pseudoislets using high-density concave microwell arrays with potential for future integration into macroencapsulation devices. Microwells with diameters between 130 and 200 µm were arranged in a hexagonal pattern with inter-well spacings of 50 or 100 µm to maximize the packing density while maintaining separation between clusters. The effects of microwell depth, diameter, and inter-well spacing on pseudoislet size, morphology, viability, and glucose-stimulated insulin secretion were systematically evaluated using reaggregated rat islet cells as a standardized and well-characterized model system under normoxic and hypoxic conditions. The optimized microwell configuration enabled efficient pseudoislet formation with a tailorable, uniform size, high viability, and preserved insulin secretory function, even at high densities. This scalable microwell array technology addresses key limitations of current pseudoislet culture and transplantation methods, thus providing a base for improved macroencapsulation device design in diabetes cell therapy.