A mainport is a large transport hub (airport or seaport) with multi-modal connections. Mainport strategic planning develops long-term plans for matching capacity and demand while staying within environmental constraints and achieving financial success. Mainport planning is difficult, since it involves many stakeholders, multiple (often conflicting) criteria, uncertainty about the future, and many potential strategic plans. Many mainport strategic plans have resulted in costly failures, due in part to the processes and tools the planners use. They often consider only a single forecast for the future, consider few alternative strategies, and exclude stakeholders, resulting in plans that quickly become obsolete and may be opposed by stakeholders. Avoiding such failures requires an integrated approach to decision support that enables collaboration among stakeholders. Such an approach requires an effective way to deal with many inputs and outputs enabling the extraction of relevant information for decisionmaking and its presentation in transparent and understandable ways. We propose a Decision Support System (DSS) that provides a way for decisionmakers and stakeholders to quickly and easily generate alternative strategic plans and evaluate them with respect to a large number of outcomes of interest for a wide range of plausible scenarios. This paper describes an architecture for such a DSS.