Lake Malawi, also known as Lake Nyasa, Lake Nyassa, or Lake Niassa after the Yao word for “lake” (officially called Niassa in Mozambique), is the most southerly lake in the Great African Rift Valley system.

The lake is about 560 km long and 75 km wide at its widest point, with a total area of approximately 29,600 km², and is bordered by Mozambique, Malawi, and Tanzania. Its outlet is the Shire River; its largest tributary is the Ruhuhu.

The lake lies in the Great African Rift Valley, a large graben caused by crustal extension. It probably formed about 40,000 years ago.

The lake water is generally warm having a surface temperature that ranges from 24 - 29 °C (75 - 84 °F) and a deep level temperature of 22 °C (71.6 °F)

 The water in lake Malawi is typically alkaline with a pH of 7.7 - 8.6

Lake Malawi is famous for its cichlids, popular in the aquarium trade. Malawi cichlids are divided into two basic groups. These are loosely referred to as the Haplochromines and the Tilapiines. Within this first group (Haplochromines) are two sub-groups.

The first sub-group consist of the open water and sand dwelling species with males usually sporting bright colors while the females show a silvery coloration with irregular black bars or various other markings.

The second sub-group is known locally and popularly as mbuna, which means rockdweller. Mbuna are smaller, generally vegetarian, and both sexes are often quite colorful, though many species are dimorphic. The second group, the Tilapiines, consists of the only substrate spawning species in the lake (Tilapia rendalli), as well as the 4 species of Chambo (Nyasalapia). The genera Maylandia and Labidochromis are popular cichlids in the international aquarium scene.

Cichlids are an important export for Malawi, but wild populations are increasingly threatened by overfishing and localized pollution.

Other wildlife resident in the lake includes crocodiles, and a large population of fish eagles which feed off the fish population.