The United States and South Korea are close allies, committed to reducing greenhouse gas emissions and home to advanced industrial economies. Pragmatic cooperation on clean energy technologies would aid their shared goals in climate mitigation, energy security, and supply chain resilience. US-South Korea cooperation should include pooling shared technological expertise, aligning market and carbon accounting rules, investing in constrained supply chains, and adopting well-targeted policies to encourage research, development, and demonstration (RD&D) projects for early-stage technologies like clean hydrogen and advanced nuclear power plants. Clean energy pragmatism requires acknowledgment of the constraints facing each country to ensure policies are properly scoped, politically durable, and revised iteratively in light of observed outcomes. Neither country can succeed in this multidecadal transition alone. With their shared industrial capacity, world-class innovators engaged in joint research and development, and properly aligned market standards, however, both countries can solidify their position to achieve 21st-century energy security and economic growth. Yet, policy design and implementation are never complete unless a problem is entirely solved, and climate change is not a problem that can be solved over a single presidential term. Political shifts hold the potential to either undo progress or help spur new action.