The Gulf of Mannar is a large shallow bay forming part of the Laccadive Sea in the Indian Ocean with an average depth of 5.8 m (19 ft). It lies between the southeastern tip of India and the west coast of Sri Lanka, in the Coromandel Coast region. The chain of low islands and reefs known as Adam’s Bridge or Rama Setu, which includes Mannar Island, separates the Gulf of Mannar from Palk Bay, which lies to the north between India and Sri Lanka. The estuaries of Thamirabarani River and Vaippar River of South India and the Malvathu Oya of Sri Lanka drain into the Gulf. The dugong is found here.

The Indian Ocean is the third-largest of the world's five oceanic divisions, covering 70,560,000 km2 (27,240,000 sq mi) or approx. 20% of the water on Earth's surface. It is bounded by Asia to the north, Africa to the west and Australia to the east. To the south it is bounded by the Southern Ocean, or Antarctica, depending on the definition in use. Along its core, the Indian Ocean has large marginal, or regional seas, such as the Andaman Sea, the Arabian Sea, the Bay of Bengal, and the Laccadive Sea.