The sea dominates Southeast Asia, covering roughly eighty per cent of its area. The region’s islands and peninsulas, wedged between the Pacific and Indian Oceans, border major arteries of communication and trade. Thus the economic and political affairs of Southeast Asia have been dominated by the sea. Today more than half of the world’s annual merchant tonnage traverses Southeast Asian waters; its oceans and seas yield vast revenues in such industries as fishing, hydrocarbon extraction, and tourism. In fact, more than sixty per cent of Southeast Asians today live in or rely economically on the maritime zones. But a variety of dangers have directly threatened to safety and freedom of navigation. These threats are transnational crime, environment degradation, non-state political violence, territorial disputes and China’s territorial claim on the resource-rich South China Sea. This paper discusses the threats to maritime security in Southeast Asia and maritime security cooperation.