Diabetes results from a reduction of pancreatic β-cells. Stimulating replication could normalize β-cell mass. However, adult human β-cells are recalcitrant to proliferation. We identified osteoprotegerin, a bone-related decoy receptor, as a β-cell mitogen. Osteoprotegerin was induced by and required for lactogen-mediated rodent β-cell replication. Osteoprotegerin enhanced β-cell proliferation in young, aged, and diabetic mice. This resulted in increased β-cell mass in young mice and significantly delayed hyperglycemia in diabetic mice. Osteoprotegerin stimulated replication of adult human β-cells, without causing dedifferentiation. Mechanistically, osteoprotegerin induced human and rodent β-cell replication by modulating CREB and GSK3 pathways, through binding Receptor Activator of NF-κB (RANK) Ligand (RANKL), a brake in β-cell proliferation. Denosumab, an FDA-approved osteoporosis drug, and RANKL-specific antibody induced human β-cell proliferation in vitro, and in vivo, in humanized mice. Thus, osteoprotegerin and Denosumab prevent RANKL/RANK interaction to stimulate β-cell replication, highlighting the potential for repurposing an osteoporosis drug to treat diabetes.