Schnurri-3, a large zinc finger protein distantly related to Drosophila Shn, is a potent and essential regulator of adult bone formation. Mice lacking Shn3 display an osteosclerotic phenotype with profoundly increased bone mass due to augmented osteoblast activity. Shn3 controls protein levels of Runx2, the principal regulator of osteoblast differentiation, by promoting its degradation. In osteoblasts, Shn3 functions as a component of a trimeric complex between Runx2 and the E3 ubiquitin ligase WWP1. This complex inhibits Runx2 function and expression of genes involved in extracellular matrix mineralization due to the ability of WWP1 to promote Runx2 polyubiquitination and proteasome-dependent degradation. Our study reveals an essential role for Shn3 as a regulator of postnatal bone mass. Compounds designed to block Shn3/WWP1 function may be possible therapeutic agents for the treatment of osteoporosis.