Sook-yeon Lee argues that Indonesia serves as an ideal partner for the United States and South Korea to advance their objectives in the Indo-Pacific region and that the three countries should expand areas for trilateral cooperation. While the bilateral South Korea-Indonesia and US-Indonesia partnerships have deepened over the past several decades, trilateral security cooperation remains unexplored due to the differing scopes of cooperation between the two bilateral relationships. Lee outlines several areas in which the three countries should expand trilateral cooperation and contribute to strengthening regional security. Due to Indonesia’s desire for neutrality and South Korea’s concerns regarding its ties with China, Lee recommends a cautious approach to initiating trilateral cooperation in maritime security and counterterrorism before gradually expanding to additional areas. Such an approach both addresses Indonesia’s immediate cooperative needs and aligns with areas that South Korea and the United States have already identified within their respective partnerships with Indonesia. However, the primary challenge facing trilateral security cooperation is maintaining mutual trust and ensuring the leaders of all three countries are committed to upholding trust as the foundation of trilateralism. Domestic politics and foreign policy considerations of the three countries further complicate efforts to enhance trilateral security cooperation. Thus, Lee recommends that the three countries strengthen strategic dialogues to understand and respect the differences in each country’s respective core interests to ensure the establishment of a durable trilateral partnership between South Korea, Indonesia, and the United States.