Defects in β-catenin regulation contribute to the neoplastic transformation of mammalian cells. Dysregulation of β-catenin can result from missense mutations that affect critical sites of phosphorylation by glycogen synthase kinase 3β (GSK3β). Given that phosphorylation can regulate targeted degradation of β-catenin by the proteasome, β-catenin might interact with an E3 ubiquitin ligase complex containing an F-box protein, as is the case for certain cell cycle regulators. Accordingly, disruption of the Drosophila F-box protein Slimb upregulates the β-catenin homolog Armadillo. We reasoned that the human homologs of Slimb – β-TrCP and its isoform β-TrCP2 (KIAA0696) – might interact with β-catenin. We found that the binding of β-TrCP to β-catenin was direct and dependent upon the WD40 repeat sequences in β-TrCP and on phosphorylation of the GSK3β sites in β-catenin. Endogenous β-catenin and β-TrCP could be coimmunoprecipitated from mammalian cells. Overexpression of wild-type β-TrCP in mammalian cells promoted the downregulation of β-catenin, whereas overexpression of a dominant-negative deletion mutant upregulated β-catenin protein levels and activated signaling dependent on the transcription factor Tcf. In contrast, β-TrCP2 did not associate with β-catenin. We conclude that β-TrCP is a component of an E3 ubiquitin ligase that is responsible for the targeted degradation of phosphorylated β-catenin.