Inconsistent efforts at international cooperation often undermined global efforts to mitigate the COVID-19 health pandemic. Pundits and scholars alike laid much of the blame for this lack of cooperation on domestic political factors, especially populist leaders. Could international relations theories have predicted this behavior? I argue that there are no off-the-shelf theories that engage populism with traditional mechanisms of international cooperation, especially cooperation facilitated by international institutions. I explore how populist sentiment, whether stemming from the public or leaders, can pose barriers to cooperation. I argue that populists are especially likely to resist cues from foreign actors; are especially reticent to delegate national sovereignty; and are especially resistant to policies that result in gains for elites and, when coupled with nationalism, foreigners. The essay concludes with suggestions for further theoretical and empirical research.