Sasthamcotta Lake or Sasthamkotta Lake, also categorized as a wetland, is the largest fresh water lake in Kerala, a state of India on the south of the West Coast. The lake is named after the ancient Sastha temple (a pilgrimage centre) located on its bank. It meets the drinking water needs of half million people of the Quilon district and also provides fishing resources. The purity of the lake water for drinking use is attributed to the presence of large population of larva called cavaborus that consumes bacteria in the lake water. The lake is a designated wetland of international importance under the Ramsar Convention since November 2002.

The lake is located at a distance of 25 km from Quilon city, which is on the northern side of Ashtamudi Lake. Thiruvananthapuram International Airport, at 105 km, is the nearest airport to Kollam. Karunagapally, at a distance of 8 km, is the closest town to the lake. A ferry service across the lake transports people between West Kallada and Sasthamkotta.

Except for an earthen embankment of 1.5 km length which separates the lake from the paddy fields on its southern side, bordering the alluvial plains of the Kallada River, all other sides of the lake are surrounded by hills which are steep and form narrow valleys. In the south and southwestern parts of the lake there are a number of smaller water bodies and waterlogged areas. The present area of the lake is 375 ha since large part of the lake is reported to have been occupied for agriculture. Rock formation of mainly archaean origin are recorded with intrusions of charnockite, biotite gneiss and dolerite dyke rocks. The Tertiary Varkala formations are observed along the coastal beds. In the valley portion, sand and silt deposits are recorded. Geomorphological divisions of the basin area of the lake comprise a) the undulating uplands which have fairly thick vegetation of mixed crops and plantations, b) the valley fills of laterite alluvium and colluvial deposits with low level areas which are intensely cultivated and thickly populated and c) the flood plains/alluvial plains of the Kallada River on the south that are mostly cultivated.

There are no visible tributaries feeding the lake but springs at the bottom of the lake are stated to be one of the source which supply water throughout the year; volume of water in the lake is estimated to be of the order of 22.4 million cum. A thick 10–20 m mantle of kaolinite rich (derivative of laterite) soil around the lake is stated to check flow into the lake and the theory is now that the lake is also rain fed; the lake water level is recorded to be higher at the end of the monsoon season. The average annual rainfall in the area is 2398 mm and mean annual temperature varies between 26.7 °C and 29.16 °C. Ground water table in the area is reported at depth of 3.89 m. After accounting for estimated evaporation loss of 5 MCM (million cubic metres) and water utilized for domestic use of 8 MCM (at a pumpage rate of 22 MLD), the two together account for about two-thirds of the inflow.

The lake water is reported to be free of common salt or other minerals and metals. The quality for surface water, interstitial water and sediments studied by World Wildlife Fund (WWF), India are briefly as noted below.

Quality for surface water (average values):

Water quality test results indicate that the lake water conforms to standards of drinking water prescribed by regulatory bodies.

Eastern shore of the lake has the insectivorous plant Drosera Sp. Vegetation is negligible and rooted plants and floating plants are unimportant and insignificant. The crops grown on the banks of the lake, apart from paddy, are the plantation crops such as cashewnut, tapioca and plantain.