Most breast cancer metastases in bone form osteolytic lesions, but the mechanisms of tumor‐induced bone resorption and destruction are not fully understood. Although it is well recognized that Wnt/β‐catenin signaling is important for breast cancer tumorigenesis, the role of this pathway in breast cancer bone metastasis is unclear. Dickkopf1 (Dkk1) is a secreted Wnt/β‐catenin antagonist. In the present study, we demonstrated that activation of Wnt/β‐catenin signaling enhanced Dkk1 expression in breast cancer cells and that Dkk1 overexpression is a frequent event in breast cancer. We also found that human breast cancer cell lines that preferentially form osteolytic bone metastases exhibited increased levels of Wnt/β‐catenin signaling and Dkk1 expression. Moreover, we showed that breast cancer cell‐produced Dkk1 blocked Wnt3A‐induced osteoblastic differentiation and osteoprotegerin (OPG) expression of osteoblast precursor C2C12 cells and that these effects could be neutralized by a specific anti‐Dkk1 antibody. In addition, we found that breast cancer cell conditioned media were able to block Wnt3A‐induced NF‐kappaB ligand reduction in C2C12 cells. Finally, we demonstrated that conditioned media from breast cancer cells in which Dkk1 expression had been silenced via RNAi were unable to block Wnt3A‐induced C2C12 osteoblastic differentiation and OPG expression. Taken together, these results suggest that breast cancer‐produced Dkk1 may be an important mechanistic link between primary breast tumors and secondary osteolytic bone metastases. © 2008 Wiley‐Liss, Inc.