Breast cancer is marked by large increases in the protein fibers around tumor cells. These fibers increase the mechanical stiffness of the tissue, which has long been used for tumor diagnosis by manual palpation. Recent research in bioengineering has led to the development of novel biomaterials that model the mechanical and architectural properties of the tumor microenvironment and can be used to understand how these cues regulate the growth and spread of breast cancer. Herein, we provide an overview of how the mechanical properties of breast tumor tissues differ from those of normal breast tissue and non-cancerous lesions. We also describe how biomaterial models make it possible to understand how the stiffness and viscosity of the extracellular environment regulate cell migration and breast cancer metastasis. We highlight the need for biomaterial models that allow independent analysis of the individual and different mechanical properties of the tumor microenvironment and that use cells derived from different regions within tumors. These models will guide the development of novel mechano-based therapies against breast cancer metastasis.