The Gulf of Mannar is a vast shallow bay that forms part of the Laccadive Sea in the Indian Ocean. It is situated in the Coromandel Coast region between the south-eastern tip of India and the West Coast of Sri Lanka.

The Gulf of Mannar is separated from Palk Bay to the north by a chain of low islands, including Mannar Island, and reefs known as Ram Setu, which are also called Adam's Bridge. The South Indian estuary of Thamirabarani River and the Sri Lankan estuary Malvathu Oya (Malvathu River) drain into the Gulf of Mannar. There are two major seaports on the Gulf, Colombo in Sri Lanka and Thoothukudi (Tuticorin) in Tamil Nadu, and both can accommodate deep-draft vessels.

There are known to be over 3,600 species of flora and fauna in the Gulf of Mannar, making it one of the richest coastal regions for marine life in Asia. Sea Turtles, Sharks, Dugongs and Dolphins are regular visitors to the Gulf, and 117 species of hard coral have been recorded here, but unfortunately the effects of pollution, over-fishing, and coral mining have been detrimental, resulting in their endangerment. There are 47 fishing villages along the coast that rely on the reef fish to feed their families, but the fish population has also declined significantly.

For at least two thousand years the Gulf of Mannar has also been known for its pearl banks of Pinctada radiata and Pinctada fucata. Pearl extraction is still carried out here while elsewhere in the world, the extraction of natural pearls is considered to be too expensive.

The Gulf of Mannar is also home to a Biosphere Reserve that covers an area of 10,500 square kilometres of ocean, 21 islets and the adjacent coastline. These islets and the coastal buffer zone is composed of beaches, estuaries, and tropical dry broadleaf forests, while the marine environments includes coral reefs, salt marshes, seaweed and sea grass communities, and mangrove forests.