The total amount exploitable hydropower resources in the Association for South East Asia Nations (ASEAN) is large providing a low-carbon development opportunity for the region through cross-border grid connection. For more than a decade several, goals and planning frameworks have been formulated and implemented but no effort has been made to assess the sustainability potentials of planned interconnections. This paper establishes a methodology to critically review the energy security, affordability and carbon emission profile of the region under deepened cross border grid connectivity scenarios. The analysis shows the planned grid connectivity is economically and technically feasible, but changes in the current institutional environment will inevitably bring low-carbon benefits of cross-border grid connectivity. Investments in a diversity of renewable energy options to the interconnected grids could enhance ASEAN energy security and meet climate goals, without having to resort to building fossil fuel powered plants. Considering the cross-borders grid connectivity and integration of renewable energy into current grid infrastructure options are capital intensive, a further coordinated approach for the ASEAN governments involved is recommended.