The adenomatous polyposis coli (APC) tumor-suppressor protein, together with Axin and GSK3β, forms a Wnt-regulated signaling complex that mediates phosphorylation-dependent degradation of β-catenin by the proteasome. Siah-1, the human homolog of Drosophila seven in absentia, is a p53-inducible mediator of cell cycle arrest, tumor suppression, and apoptosis. We have now found that Siah-1 interacts with the carboxyl terminus of APC and promotes degradation of β-catenin in mammalian cells. The ability of Siah-1 to downregulate β-catenin signaling was also demonstrated by hypodorsalization of Xenopus embryos. Unexpectedly, degradation of β-catenin by Siah-1 was independent of GSK3β-mediated phosphorylation and did not require the F box protein β-TrCP. These results indicate that APC and Siah-1 mediate a novel β-catenin degradation pathway linking p53 activation to cell cycle control.