Islet transplantation is a cellular replacement therapy to treat severe diabetes mellitus, but its clinical outcome is unsatisfactory at present. One factor in clinical success of this therapy is selection of the most appropriate transplantation site. In this review, we review evidence showing the advantages of the spleen as a transplantation site for islets. The spleen has been studied for a long time as a candidate site for islet transplantation. Its advantages include physiological insulin drainage and regulation of immunity. Recently it has also been shown that the spleen contributes to the regeneration of transplanted islets and that splenic stem cells have the potential to differentiate into islet cells. The spleen also has some disadvantages associated with the transplantation procedure itself (bleeding, thrombosis and splenic infarction). The efficacy of transplantation is not as high as that obtained with intraportal transplantation, which is the current representative method of clinical islet transplantation. Safer and more effective methods of islet transplantation need to be established before the spleen can be effectively used in the clinic to support the engraftment of multiple transplanted islets.