The osteoblast-specific hormone osteocalcin was found to regulate glucose metabolism, fat mass and β-cell proliferation in mice. Here we investigate the effect of decarboxylated osteocalcin on human β-cell function and mass in culture and in vivo using a NOD-scid mouse model. We found that decarboxylated osteocalcin at dose ranges from 1.0-15ng/ml significantly augmented insulin content and enhanced human β-cell proliferation of cultured human islets. This was paralleled by increased expression of SUR1 protein; a marker of beta-cell differentiation and a component of the insulin secretory apparatus. Moreover, in a NOD-scid mouse model, systemic administration of decarboxylated osteocalcin at 4.5ng/hr significantly augmented production of human insulin and C-peptide from the grafted human islets. Finally, histological staining of the human islet grafts showed that the improvement in the β-cell function was attributable to an increase in β-cell mass as a result of β-cell proliferation indicated by Ki67 staining as suggested by the histological results together with the increased β-cell number and decreased α-cell number data obtained using laser scanning cytometry. Our data for the first time show decarboxylated osteocalcin-enhanced β-cell function in human islets and support future exploitation of decarboxylated osteocalcin-mediated β-cell regulation for developing useful clinical treatments for patients with diabetes.