Dysregulation of Wnt–β-catenin signaling disrupts axis formation in vertebrate embryos and underlies multiple human malignancies. The adenomatous polyposis coli (APC) protein, axin, and glycogen synthase kinase 3β form a Wnt-regulated signaling complex that mediates the phosphorylation-dependent degradation of β-catenin. A protein phosphatase 2A (PP2A) regulatory subunit, B56, interacted with APC in the yeast two-hybrid system. Expression of B56 reduced the abundance of β-catenin and inhibited transcription of β-catenin target genes in mammalian cells and Xenopusembryo explants. The B56-dependent decrease in β-catenin was blocked by oncogenic mutations in β-catenin or APC, and by proteasome inhibitors. B56 may direct PP2A to dephosphorylate specific components of the APC-dependent signaling complex and thereby inhibit Wnt signaling.