Here, rational engineering of doxorubicin prodrug loaded peptide-targeted liposomal nanoparticles to selectively target metastatic breast cancer cells in vivo is described. Glucose-regulated protein 78 (GRP78), a heat shock protein typically localized in the endoplasmic reticulum in healthy cells, has been identified to home to the cell surface in certain cancers, and thus has emerged as a promising therapeutic target. Recent reports indicated GRP78 to be expressed on the cell surface of an aggressive subpopulation of stem-like breast cancer cells that exhibit metastatic potential. In this study, a targeted nanoparticle formulation with a GRP78-binding peptide (Kd of 7.4 ± 1.0 μM) was optimized to selectively target this subpopulation. In vitro studies with breast cancer cell lines showed the targeted nanoparticle formulation (TNPGRP78pep) achieved enhanced cellular uptake, while maintaining selectivity over the control groups. In vivo, TNPGRP78pep loaded with doxorubicin prodrug was evaluated using a lung metastatic mouse model and demonstrated inhibition of breast cancer cell seeding to lungs down at the level of negative control groups. Combined, this study established that specific-targeting of surface GRP78 expressing a subpopulation of aggressive breast cancer cells was able to inhibit breast cancer metastasis to lungs, and underpinned the significance of GRP78 in breast cancer metastasis.