Abstract: Why might bilateral antagonism prove resilient to incentives for improvement from multilateral cooperation with shared third parties? Prominent theories predict that two actors with a record of cooperation in multilateral contexts would cooperate bilaterally as well; multilateralism is generally thought harder than bilateralism and provides opportunities for exposure and socialization that can facilitate preference convergence. This article presents Japan–South Korea relations as a deviant case for such expectations in the cooperation literature. Rather than think of multilateral cooperative contexts as “bridges” that facilitate closer, positive relations between actors, this case shows that multilateralism can instead be a “buffer” between two actors with negatively valenced ties, mediating bilateral friction sufficient to facilitate functional cooperation while insulating antagonistic national discourses or bilateral policies from pressures for change. In the case of Japan–South Korea relations, a pattern of simultaneous cooperation (with shared third parties) and friction (in bilateral interactions) over the same period illustrates a potential buffering logic of multilateralism; the multiparty context diffuses accountability for cooperative behavior that might otherwise generate domestic audience costs and allows policy elites to frame cooperation in a way that downplays or ignores the other.