Vembanad (Malayalam: [ʋeːmbɐnaːɖə̆]) is the longest lake in India, and the largest in the state of Kerala. The lake has an area of 2,033 square kilometres (785 sq mi) and a maximum length of 96.5 km (60 mi).

Spanning three districts in the state of Kerala, it is known as Vembanad Lake in Kottayam, Vaikom, Changanassery, Punnamada Lake in Alappuzha, Punnappra, Kuttanadu and Kochi Lake in Kochi. Several groups of small islands including Vypin, Mulavukad, Maradu, Udayamperoor, Vallarpadam, and Willingdon Island are located in the Kochi Lake portion. Kochi Port is built around Willingdon Island and Vallarpadam island.

Kuttanad, also known as The Rice Bowl of Kerala, has the lowest altitude in India, and is also one of the few places in the world where cultivation takes place below sea level. Kuttanad lies on the southern portion of Vembanad. The Nehru Trophy Boat Race is conducted in a portion of the lake. High levels of pollution have been noticed at certain hotspots of the Vembanad backwaters. The Government of India has identified the Vembanad wetland under the National Wetlands Conservation Programme.

The Vembanad lake is 96.5 km (60 mi) long, making it the longest lake in India. The Vembanad wetland system covers an area of over 2,033.02 km² (784.95 sq mi) thereby making it the largest wetland system in India. Of this, an area of 398.12 km² (153.71 sq mi) is located below the mean sea level (MSL) and a total of 763.23 km² (294.68 sq mi) area is located below 1 metre (3 ft 3 in) MSL. The lake is bordered by Alappuzha, Kottayam, and Ernakulam districts. It is situated at sea level, and is separated from the Laccadive Sea by a narrow barrier island. Canals link the lake to other coastal lakes in the north and south. The lake surrounds the islands of Pathiramanal, Perumbalam and Pallippuram. The Vembanad Lake is approximately 14 km (8.7 mi) wide at its widest point.

The lake is a part of the Vembanad-Kol wetland system which extends from Alappuzha in the south to Azheekkode in the north, making it by far India's longest lake at just over 96.5 km (60 mi) in length. The lake is fed by 10 rivers flowing into it including the six major rivers of central Kerala namely the Achenkovil, Manimala River, Meenachil River, Muvattupuzha River, Pamba River and Periyar River. The total area drained by the lake is 15,770 km² (6,090 sq mi), which accounts for 40% of the area of Kerala. Its annual surface runoff of 21,900 mm (860 in) accounts for almost 30% of the total surface water resource of the state.

The most popular location on the shores of the lake is the Kumarakom Tourist Village situated on the east coast of the lake. The Kumarakom Bird Sanctuary is located on the northern fringes of Kumarakom village. The Vembanad Wetland system was included in the list of wetlands of international importance, as defined by the Ramsar Convention for the conservation and sustainable utilization of wetlands in 2002. It is the largest of the three Ramsar Sites in the state of Kerala. Vembanad Lake has been heavily reclaimed over the course of the past century with the water spread area reducing from 290.85 km² (112.30 sq mi) in 1917 to 227 km² (88 sq mi) in 1971 and 213.28 km² (82.35 sq mi) in 1990. In the same period almost 63.62 km² (24.56 sq mi) of erstwhile water spread were reclaimed primarily for the formation of polders and to enlarge the extent of the Wellington island of Cochin port. The lake faces a major ecological crisis and has reduced to 37 percent of its original area, as a result of land reclamation.

A unique characteristic of the lake is the 1,252 metres (4,108 ft)-long Thanneermukkom salt water barrier constructed as a part of the Kuttanad Development Scheme to prevent tidal action and intrusion of salt water into the Kuttanad low-lands. It is the largest mud regulator in India and essentially divides the lake into two parts – one with perennial brackish water and the other with fresh water from rivers draining into the lake. This barrier has helped farmers in Kuttanad by freeing the area of salinity and allowing them an additional crop in the dry season.

The Thanneermukkom barrier is located at one of the narrower parts of the Vembanad Lake. Only two-thirds of the original number of gates are opened in July to release flood flow. These gates remain closed until mid-November. The main drawback of the structure has been the loss of opportunity for fish and prawns to migrate upstream, and also an increase in weed growth in the upstream, severely restricting the natural flushing of pollutants. The Thanneermukkom bund has created ecological problems, primarily, the rampant propagation of the water hyacinth in fresh water.^{[citation needed]}