Integrins are important adhesion receptors in all Metazoa that transmit conformational change bidirectionally across the membrane. Integrin α and β subunits form a head and two long legs in the ectodomain and span the membrane. Here, we define with crystal structures the atomic basis for allosteric regulation of the conformation and affinity for ligand of the integrin ectodomain, and how fibrinogen-mimetic therapeutics bind to platelet integrin α_IIbβ₃. Allostery in the β₃ I domain alters three metal binding sites, associated loops and α1- and α7-helices. Piston-like displacement of the α7-helix causes a 62° reorientation between the β₃ I and hybrid domains. Transmission through the rigidly connected plexin/semaphorin/integrin (PSI) domain in the upper β₃ leg causes a 70 Å separation between the knees of the α and β legs. Allostery in the head thus disrupts interaction between the legs in a previously described low-affinity bent integrin conformation, and leg extension positions the high-affinity head far above the cell surface.