β-catenin plays an important role in development and homeostasis. Deregulated β-catenin is involved in oncogenesis. In this study, we found that β-catenin can physically complex with NF-κB, resulting in a reduction of NF-κB DNA binding, transactivation activity, and target gene expression. Repressed NF-κB activity is found in human colon cancer cells in which β-catenin is activated. Importantly, activated β-catenin was found to inhibit the expression of NF-κB target genes, including Fas and TRAF1. Furthermore, a strong inverse correlation was identified between the expression levels of β-catenin and Fas in colon and breast tumor tissues, suggesting that β-catenin regulates NF-κB and its targets in vivo. Thus, β-catenin may play an important role in oncogenesis through the crossregulation of NF-κB.