Raf kinase inhibitor protein (RKIP) has been shown to be a metastasis suppressor in many kinds of malignant tumors. But its function in breast cancer was not yet clarified completely. We detected RKIP expression in clinical samples of primary breast cancer, breast cancer metastases, and in different breast cancer cells. Compared with the normal breast epithelia, benign breast epithelia, or in situ ductal carcinoma, the expression level of RKIP is decreased in invasive carcinoma and significantly reduced or lost in the metastasis lymph node matched to the invasive carcinoma. To explore the potential role of RKIP in breast cancer metastasis, we studied the effect of RKIP on the malignant phenotypes of the breast cancer cells with ectopically overexpression or knockdown of RKIP. Cell proliferation, soft-agar colony formation, in vitro adhesion assay, invasion, and migation assays were done to examine the malignant phenotypes of the transfected cells. Consequently, RKIP has no effect on in vitro proliferation rate or colony-forming ability of MDA-MB-435 cells. In vitro cell invasion and migration assays indicated that the RKIP expression was inversely associated with the invasiveness of MDA-MB-435 cells. Consistent with these results, in the orthotopic murine models, we observed that overexpression of RKIP in breast cancer cells impaired invasiveness and metastasis, whereas down-regulation of RKIP expression promoted invasiveness and metastasis. These results indicate that RKIP is a metastasis suppressor gene of human breast cancer. (Mol Cancer Res 2009;7(6):832–40)