Bone morphogenetic protein 9 (BMP9) is a member of the TGF /BMP superfamily, and we have demonstrated that it is one of the most potent BMPs to induce osteoblast differentiation of mesenchymal stem cells (MSCs).Here, we sought to investigate if canonical Wnt/-catenin signaling plays an important role in BMP9-induced osteogenic differentiation of MSCs.Wnt3A and BMP9 enhanced each others ability to induce alkaline phosphatase (ALP) in MSCs and mouse embryonic fibroblasts (MEFs).Wnt antagonist FrzB was shown to inhibit BMP9-induced ALP activity more effectively than Dkkl, whereas a secreted form of LPR5 or LRP6 exerted no inhibitory effect on BMP9-induced ALP activity.-Catenin knockdown in MSCs and MEFs diminished BMP9-induced ALP activity, and led to a decrease in BMP9-induced osteocalcin reporter activity and BMP9-induced expression of late osteogenic markers.