Paramyxoviruses—including important pathogens like parainfluenza, measles, and Nipah viruses—use a receptor binding protein [hemagglutinin-neuraminidase (HN) for parainfluenza] and a fusion protein (F), acting in a complex, to enter cells. We use cryo–electron tomography to visualize the fusion complex of human parainfluenza virus 3 (HN/F) on the surface of authentic clinical viruses at a subnanometer resolution sufficient to answer mechanistic questions. An HN loop inserts in a pocket on F, showing how the fusion complex remains in a ready but quiescent state until activation. The globular HN heads are rotated with respect to each other: one downward to contact F, and the other upward to grapple cellular receptors, demonstrating how HN/F performs distinct steps before F activation. This depiction of viral fusion illuminates potentially druggable targets for paramyxoviruses and sheds light on fusion processes that underpin wide-ranging biological processes but have not been visualized in situ or at the present resolution.