Wnts are secreted signaling molecules that can transduce their signals through several different pathways. Wnt-5a is considered a noncanonical Wnt as it does not signal by stabilizing β-catenin in many biological systems. We have uncovered a new noncanonical pathway through which Wnt-5a antagonizes the canonical Wnt pathway by promoting the degradation of β-catenin. This pathway is Siah2 and APC dependent, but GSK-3 and β-TrCP independent. Furthermore, we provide evidence that Wnt-5a also acts in vivo to promote β-catenin degradation in regulating mammalian limb development and possibly in suppressing tumor formation.