The cytoskeleton, integrin-mediated adhesion, and substrate stiffness control a common set of cell functions required for development and homeostasis that are often deranged in cancer. The connection between these mechanical elements and chemical signaling processes is not known. Here, we show that α5β1 integrin switches between relaxed and tensioned states in response to myosin II-generated cytoskeletal force. Force combines with extracellular matrix stiffness to generate tension that triggers the integrin switch. This switch directly controls the α5β1-fibronectin bond strength through engaging the synergy site in fibronectin and is required to generate signals through phosphorylation of focal adhesion kinase. In the context of tissues, this integrin switch connects cytoskeleton and extracellular matrix mechanics to adhesion-dependent motility and signaling pathways.