Breast cancer is a leading cause of death in women globally. It is not a single disease, but it is a collection of diseases that have different histopathological features, genetic and genomic variability, and varied prognostic outcomes. Our in vitro data showed that MSCs from human placenta showed the phenotypic and functional characteristics that can modulate the functions of breast cancer cell lines. Therefore, in this study, we will examine the anti-cancer activity of placental MSCs in vivo using an animal model of breast cancer. This study will lead to the development of a novel stem cell-based therapy that can benefit millions of patients affected by breast cancer in Saudi Arabia and in the world.