Updated project metadata. Exercise benefits the human body in many ways. Irisin is secreted by muscle, increased with exercise, and conveys many physiological benefits, including improved cognition and resistance to neurodegeneration. Irisin acts via αV integrins; however, a mechanistic understanding of how small polypeptides like irisin can signal through integrins is poorly understood. Using mass spectrometry and cryo-EM, we demonstrate that extracellular heat-shock protein 90α (eHsp90α) is secreted by muscle with exercise and acts as a required cofactor that “opens” the integrin αVβ5 structure to allows for high affinity irisin binding and signaling through an eHsp90α/αV/β5 complex. By including hydrogen/deuterium exchange data, we generate and experimentally validate a 2.98 Å RMSD irisin/αVβ5 complex docking model. Irisin binds very tightly to an alternative interface on αVβ5 distinct from that involved in its interaction with known ligands. These data together elucidate a non-canonical mechanism by which a small polypeptide hormone like irisin can function through integrins.