Significance Mechanical interactions between cells and the ECM critically regulate cell function, including growth and migration. By measuring forces exerted by breast tumor cells embedded within collagen matrices, we reveal a positive mechanical cross-talk between the cell and ECM: cells pulling onto collagen fibers align and stiffen the matrices, and stiffer collagen matrices promote greater cell force generation. Our work highlights the importance of strain-induced fiber alignment in mediating cell–ECM interaction within a 3D architecture. The basic force regulation principle uncovered here can be extended to understand the tissue-stiffening processes occurring in many diseases, such as tumor progression and fibrosis, and better design biomaterial scaffolds to control cell behavior in tissue engineering applications.