Wellington Koo (1888–1985) is a prominent jurist and diplomat in the Republic of China. The article examines his contribution to international legal order and Chinese diplomacy. It argues that Koo’s involvement in international jurisprudence and Chinese foreign policy validated the universality of international law and reinforced the normative foundation of global governance. First, the article sheds light on the historical background of Republican China and assesses Koo’s decisions as a judge in the International Court of Justice, including high-profile disputes over South West Africa and the Temple of Preah Vihear. Second, it examines Koo’s impact on Chinese diplomacy based on assertive legalism. In particular, it focuses on Koo’s efforts to deal with the Japanese aggression to Shandong and Manchuria, to end China’s unequal treaties and to contribute to the forming of the United Nations. Hence, Koo left a rich legacy in the evolution of modern Asia’s approach to international law.