Osteosarcoma (OS) is the most common type of bone tumor in children and adults. The expression of APCDD1, a Wnt antagonist, was reduced in OS tissues and cells compared to adjacent normal tissue and osteoblast cells, respectively. Mechanistically, this was due to increased levels of methylation in the promoter region of the APCDD1 gene. Consistently, the DNA methyltransferase inhibitor 5-AZA-dC, reduced DNA methylation in the APCDD1 promoter, and restored APCDD1 expression in OS tissue and cells. Moreover, DNMT3a, but not DNMT1 or DNMT3b, was the major DNA methyltransferase that facilitated hyper-methylation of DNA in the APCDD1 promoter, thus reducing APCDD1 mRNA levels in OS tissues. Importantly, ectopic expression of APCDD1 suppressed activity of the Wnt/beta-Catenin signaling pathway in OS cells and inhibited their invasion and reversed their EMT-like properties, while depletion of APCDD1 promoted invasion and metastasis of osteosarcoma cells in vitro and in vivo. That APCDD1 modulates the gene expression of Wnt- and EK-related signaling molecules at the cap stage of tooth development, and is involved in tooth cusp patterning by modulating the epithelial rearrangement in the IEE. In hair follicle cells APCDD1 inhibits the canonical WNT/beta-Catenin pathway and its inactivation is associated with an autosomal dominant form of hair loss. APCDD1 sustains the expression and activation of beta-Catenin.