Rapid increases in electricity consumption in Southeast Asia caused by rising living standards and population raise concerns about energy security, affordability and environmental sustainability. In this study, the role of short-term off-river energy storage (STORES) in supporting 100% renewable electricity in Southeast Asia is investigated. Large-scale integration of off-river, closed-loop pumped hydro storage is a new approach to providing system flexibility facilitating high penetration of variable renewable energy in electricity systems. The features of STORES include large storage potential, high technology maturity and a long service life. Energy generation, storage and transmission are co-optimised based on long-term, high-resolution chronological energy data. A comparative analysis is undertaken between the scenarios with and without an intercontinental Asia-Pacific Super Grid. The results show that, with support provided by STORES, the Southeast Asian electricity industry can achieve very high penetration (78%-97%) of domestic solar and wind energy resources. The levelised costs of electricity range from 55 to 115 U.S. dollars per megawatt-hour based on 2020 technology costs. In the Super Grid scenarios, the costs change by -4% to +7% while the storage requirements reduce by 50%-89%. Renewable energy supported by STORES can be a cost-effective solution for Southeast Asia's energy transition, delivering long-term, substantial environmental benefits.