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.