The S of UNESCO stands for science, and provided some impetus for UNESCO's creation. This essay critically examines the ways in which science diplomacy at the multilateral level was mobilized to produce UNESCO's vision for a ‘culture of peace’. Underlying UNESCO's vision was the diplomacy of scientific humanism, best articulated in the words and person of its first Director‐General Julian Huxley, himself a scientist. The scientific method was the axiom that would bring diverse communities together. After examining the humanism of science diplomacy, this essay outlines two major effects of UNESCO's science diplomacy: science diplomacy networks and programs. The S of UNESCO contributed to global beliefs in multilateral diplomacy as a way of resolving conflicts, especially in further propagating the use of scientific evidence in multilateral negotiations. The essay also outlines the limits of these science networks, institutions, and scientific evidence in multilateral diplomacy although science may well be the unsung ‘hero’ of an otherwise highly politicized UNESCO.