The convergence of human, animal and environmental health crises has exposed the limitations of fragmented governance structures. One Health faces a critical implementation gap: the persistent silos between scientific disciplines, sectors and sovereign states. This perspective argues that global health diplomacy provides the essential mechanism for designing effective One Health governance. We propose a three-part diplomatic agenda: constructing science–policy interfaces, negotiating institutional architectures, and embedding equity and solidarity as core governance principles. Without deliberate diplomatic engagement, One Health risks remaining an aspirational concept rather than an operational reality.