South Korea, Japan, and the United States' trilateral partnership has expanded to include collaboration on economic security, including on critical minerals supply chains (CMSCs). This report offers analysis, tools, and recommendations to strengthen South Korea's CMSCs and economic security through trilateral cooperation and collaboration among South Korea, the United States, and Japan. Included in the report are (1) analysis of the trio's CMSC vulnerabilities and four industry case studies on cobalt, gallium, molybdenum, and tungsten; (2) summaries of the key organizations and governance leading critical minerals efforts, national legislation, policy tactics and tasks, and available financial mechanisms in each country, along with a supply chain net assessment tool and analysis to evaluate supply chain securitization efforts; (3) case studies on trilateral engagement with India and Mongolia and political alignment metrics to evaluate potential partnerships with mineral-rich countries; and (4) opportunities for and barriers to trilateral cooperation. Although barriers exist, opportunities to secure South Korea's supply chains through trilateral cooperation include trade policy and sectoral trade agreements, steering the Minerals Security Partnership (MSP) into new directions, and joint stockpiling initiatives, such as mineral swap agreements. South Korea can improve its supply chain vulnerabilities by expanding its concepts of economic security to include securing raw material inputs for its defense industrial base. This report should be of interest to South Korean, Japanese, and U.S. policymakers; to researchers of Indo-Pacific and East Asian security, economic security, techno-economic competition, supply chain resilience, and related policy; and to the private sector.