Plant steroid hormones, known as brassinosteroids (BRs), signal through a plasma membrane localized receptor kinase BRI1. We identified bes1, a semidominant suppressor of bri1, which exhibits constitutive BR response phenotypes including long and bending petioles, curly leaves, accelerated senescence, and constitutive expression of BR-response genes. BES1 accumulates in the nucleus in response to BRs. BES1 is phosphorylated and appears to be destabilized by the glycogen synthase kinase-3 (GSK-3) BIN2, a negative regulator of the BR pathway. These results establish a signaling cascade for BRs with similarities to the Wnt pathway, in which signaling through cell surface receptors leads to inactivation of a GSK-3 allowing accumulation of a nuclear protein that regulates target gene expression.