Citizen distrust towards the rival country is a defining feature of protracted international rivalries, undermining meaningful cooperation that can lead to mutual benefits. How might governments establish a public opinion base that is more supportive of cooperation with the rival country? We argue that information about ongoing environmental cooperation with the rival country makes citizens more supportive of non-environmental cooperation by changing their beliefs about the rival’s trustworthiness and facilitating extrinsic reciprocity. We investigate this potential micro-level cooperation spillover in the context of the longstanding rivalry between Japan and South Korea. Our survey-based experiment finds that information about cooperation on marine plastic pollution shapes the public’s willingness to cooperate in terms of the economy and security. Our findings have important policy implications for conflict-ridden areas of the world beyond East Asia that increasingly face common environmental challenges.