Juvenile hemochromatosis is an iron-overload disorder caused by mutations in the genes encoding the major iron regulatory hormone hepcidin (HAMP) and hemojuvelin (HFE2). We have previously shown that hemojuvelin is a co-receptor for bone morphogenetic proteins (BMPs) and that BMP signals regulate hepcidin expression and iron metabolism. However, the endogenous BMP regulator(s) of hepcidin in vivo is unknown. Here we show that compared with soluble hemojuvelin (HJV.Fc), the homologous DRAGON.Fc is a more potent inhibitor of BMP2 or BMP4 but a less potent inhibitor of BMP6 in vitro. In vivo, HJV.Fc or a neutralizing antibody to BMP6 inhibits hepcidin expression and increases serum iron, whereas DRAGON.Fc has no effect. Notably, Bmp6-null mice have a phenotype resembling hereditary hemochromatosis, with reduced hepcidin expression and tissue iron overload. Finally, we demonstrate a physical interaction between HJV.Fc and BMP6, and we show that BMP6 increases hepcidin expression and reduces serum iron in mice. These data support a key role for BMP6 as a ligand for hemojuvelin and an endogenous regulator of hepcidin expression and iron metabolism in vivo.