Type 1 diabetes mellitus (T1D) is characterized by the autoimmune destruction of pancreatic beta cells, leading to insulin deficiency and necessitating lifelong external insulin administration. The transplantation of allogenic islets is a promising therapeutic approach, whereby their macro-encapsulation offers immune protection but restricts oxygenation after transplantation. This study addresses the challenge of oxygen supply by developing a spatially structured co-culture system using bioprinting, in which both pancreatic islets and the photosynthetically active microalga Scenedesmus sp. are embedded in alginate-based hydrogels. Key environmental parameters for long-term co-cultivation were developed and systematically optimized: red light illumination was identified as non-detrimental to islet viability and function while supporting microalgal photosynthesis at the same time, and a co-culture medium was formulated to fulfill the metabolic requirements of both cell types. In direct co-culture experiments under hypoxic conditions, microalgae generated sufficient oxygen to maintain normoxic conditions, thereby preserving islet viability and glucose-stimulated insulin secretion over several days. The results demonstrate that spatially organized bioprinting enables the close proximity of islets and microalgae, facilitating effective oxygen transfer in vitro. This work establishes a robust framework for functional mammalian–microalgae co-cultures, optimizing conditions to reliably maintain cell health and function through photosynthetically generated oxygen.