Lake Malawi, also known as Lake Nyasa in Tanzania and Lago Niassa in Mozambique, (Swahili: Ziwa Nyasa) is an African Great Lake and the southernmost lake in the East African Rift system, located between Malawi, Mozambique and Tanzania.

It is the fourth largest freshwater lake in the world by volume, the ninth largest lake in the world by area and the third largest and second deepest lake in Africa. Lake Malawi is home to more species of fish than any other lake in the world, including at least 700 species of cichlids. The Mozambique portion of the lake was officially declared a reserve by the Government of Mozambique on June 10, 2011, while in Malawi a portion of the lake is included in Lake Malawi National Park.

Lake Malawi is a meromictic lake, meaning that its water layers do not mix. The permanent stratification of Lake Malawi's water and the oxic-anoxic boundary are maintained by moderately small chemical and thermal gradients.

Lake Malawi is between 560 kilometres (350 mi) and 580 kilometres (360 mi) long, and about 75 kilometres (47 mi) wide at its widest point. The lake has a total surface area of about 29,600 square kilometres (11,400 sq mi). The lake is 706 m (2,316 ft) at its deepest point, located in a major depression in the north-central part. Another smaller depression in the far north reaches a depth of 528 m (1,732 ft). The southern half of the lake is shallower; less than 400 m (1,300 ft) in the south-central part and less than 200 m (660 ft) in the far south.

The lake has shorelines on western Mozambique, eastern Malawi, and southern Tanzania. The largest river flowing into it is the Ruhuhu River, and there is an outlet at its southern end, the Shire River, a tributary that flows into the Zambezi River in Mozambique. Evaporation accounts for more than 80% of the water loss from the lake, considerably more than the outflowing Shire River. The outflows from Lake Malawi into the Shire River are vital for the economy as the water resources support hydropower, irrigation and downstream biodiversity.

Concerns have been raised over the future climate change impacts of Lake Malawi due to the recent decline in lake levels and the overall drying trend. The climate in the lake region is already experiencing changes, with the temperatures predicted to increase throughout the country.

The lake is about 350 kilometres (220 mi) southeast of Lake Tanganyika, another of the great lakes of the East African Rift.

The Lake Malawi National Park is located at the southern end of the lake.