Background & Aims: Although the Hedgehog (Hh) pathway regulates development and progression of several types of cancer, its involvement in colon cancer remains unclear. We aimed to clarify the roles of Hh signaling in intestinal tumorigenesis. Methods: We studied expression of the Hh signaling components in the intestinal tumors of Apc+/Δ716 mouse, a model for familial adenomatous polyposis. We used small interfering RNAs against Smoothened (SMO), which encodes the major signal transducer of the Hh pathway, to knockdown SMO expression and explore its function in human colon cancer cell lines. We also compared the intestinal tumor phenotypes of Apc+/Δ716Smo+/- mice with those of Apc+/Δ716 mice. Results: Expression of Smo was markedly increased in the intestinal adenoma epithelium of Apc+/Δ716 mice. Importantly, SMO knockdown in human colon cancer cell lines suppressed proliferation in culture; cells arrested at the G1/S phase. Furthermore, Apc+/Δ716Smo+/- mice had decreased numbers of polyps in the large size class (Φ ≥ 1-2 mm) and recessed polyp morphology, accompanied by reduced proliferation of the tumor epithelial cells. Unexpectedly, reduced expression of Smo suppressed β-catenin-dependent transcription, rather than Hh-responsive Gli-dependent transcription. Interestingly, SMO knockdown reduced protein levels of active β-catenin and induced its nuclear exclusion. Conclusions: Smo contributes to intestinal tumorigenesis by increasing Wnt signaling. SMO might be a good therapeutic target for patients with colorectal polyps and carcinomas, even in the absence of Hh signal activation.