The Gulf of Mannar is a large shallow bay, a part of the Lakshadweep Sea. It lies between the southeastern tip of India and the west coast of Sri Lanka. The estuaries of the river Thamirabarani of south India and the Malvathu Oya (Malvathu River) of Sri Lanka drain into the Gulf of Mannar.

Geological evidence suggests that in ancient times India and Sri Lanka were connected by land. An 18-miles (30 km) long isthmus composed of limestone shoals, and coral reefs, popularly known as Adam’s Bridge or Rama’s Bridge or Ramsethu, lies between the Rameswaram Island, off the southeastern coast of Tamil Nadu, India, and the Mannar Island, off the northwestern coast of Sri Lanka. Adam’s Bridge separates the Gulf of Mannar in the southwest from the Palk Strait in the northeast. The sea in the area is very shallow, only three to 30 feet (1 to 10 metres) deep in places, and hinders navigation. Some of the sandbanks are dry. Some claim that up to the 15th century, Adam’s Bridge was completely above sea level and people travelled between India and Sri Lanka on foot. The bridge they say was breached, fissured and the channel deepened by storms when a cyclone devastated the region in 1480. The bed of the Pearl Fishery Coast in the Gulf of Mannar is a fertile breeding ground for pearl oysters. There were two distinct fisheries in the Gulf of Mannar - one on the south Indian coast, the other on the northwestern Sri Lankan coast.

On the Indian side of the Gulf of Mannar, the Pearl Fishery Coast of southern India extended along the Coromandel Coast from Thoothukudi (Tuticorin) to Kanyakumari (Cape Comorin). This fishery coast has been known in different periods of time in various languages as the Cholamandalam coast, Colkhic Gulf, Comorin coast, Coromandel coast, Fishery Coast, Kuru-Mandala coast, Ma’bar coast, Paralia, Pescaria, Fishery coast, Tirunelveli coast, Madura coast, etc. The coast took its name from the presence of natural pearls in the bed which is a fertile breeding ground for pearl oysters.

The pearl banks on the Sri Lankan side of the Gulf of Mannar stretch from the island of Mannar, off the northwestern tip of Sri Lanka, south to Chilaw.