Prostate cancer is the most prevalent cancer in men and metastatic spread to bone is detected in up to 80% of the patients with advanced disease. Cell surface oncoproteins are attractive therapeutic targets, readily accessible to antibodies and other membrane impermeable protein/peptide-based anticancer agents. Cripto is a GPI-anchored cell surface/secreted oncoprotein that plays important roles in embryogenesis, stem cell maintenance and tumour progression. GRP78 is a HSP70 family member that binds Cripto at the cell surface. We recently found that Cripto and GRP78 are both highly expressed in human castration resistant prostate cancer (PCa), but not in androgen-dependent tumours. We investigated if Cripto/GRP78 signalling promotes the aggressive, stem cell-like PCa phenotype associated with castration resistance and bone metastasis. To mimic the endosteal metastatic niche, highly metastatic human PC-3M-Pro4luc2 prostate cancer cells were cultured with primary human osteoblasts. We found that the presence of human osteoblasts reduces the proliferation of PC-3M-Pro4luc2 cells and results in induction of the E-Cadherin repressor ZEB1, causing the PCa cells to acquire a more mesenchymal, invasive phenotype as reflected by their reduced E-Cadherin/Vimentin ratio. Co-culture of PC-3M-Pro4luc2 cells with osteoblasts also greatly increased the ALDHhigh/ALDHlow ratio indicating an increase in the size of the metastatic stem/progenitor cell population. This increase in the ALDHhigh subpopulation corresponded to enhanced Cripto and GRP78 expression and stable knockdown of Cripto or GRP78 reduced PC-3M-Pro4luc2 proliferation and clonogenicity, and decreased the size of the metastasis-initiating ALDHhigh subpopulation. Finally, we used zebrafish as a model system for measuring tumour cell dissemination and metastasis and found that Cripto knockdown in PC-3M-Pro4luc2 cells led to a significant reduction in metastatic tumour burden. In conclusion, our findings point to a potential role for Cripto and GRP78 in driving metastatic, therapy-resistant phenotype and suggest that targeting the Cripto/GRP78 pathway may have significant therapeutic potential.