Lake Atitlán (Spanish: Lago de Atitlán, [atiˈtlan]) is a lake in the Guatemalan Highlands of the Sierra Madre mountain range. The lake is located in the Sololá Department of southwestern Guatemala. It is known as the deepest lake in Central America.

Atitlán means "between the waters". In the Nahuatl language, "atl" is the word for water, and "titlan" means between. The "tl" at the end of the word "atl" is dropped when the lexeme is combined with suffixes (because it is a grammatical suffix), to which is added the -ti- connector for certain adverbs, and the -tlan suffix which means "in between, beside, near" and the elements are combined to form "Atitlán".

The lake has a maximum depth of about 340 metres (1,120 ft) and an average depth of 154 metres (505 ft). Its surface area is 130.1 km² (50.2 sq mi). It is approximately 18 km × 8 km (11.2 mi × 5.0 mi) with around 20 km³ (4.8 cu mi) of water. Atitlán is an endorheic lake, fed by two nearby rivers and not draining into the ocean. It is shaped by deep surrounding escarpments and three volcanoes on its southern flank. The lake basin is volcanic in origin, filling an enormous caldera formed by a supervolcanic eruption 79,500 years ago. The culture of the towns and villages surrounding Lake Atitlán is influenced by the Maya people. The lake is about 50 kilometres (31 mi) west-northwest of Antigua. It should not be confused with the smaller Lake Amatitlán.

Lake Atitlán is renowned as one of the most beautiful lakes in the world, and is one of Guatemala's most important national and international tourist attractions. German explorer and naturalist Alexander von Humboldt called it "the most beautiful lake in the world," and Aldous Huxley famously wrote of it in his 1934 travel book Beyond the Mexique Bay: "Lake Como, it seems to me, touches on the limit of permissibly picturesque, but Atitlán is Como with additional embellishments of several immense volcanoes. It really is too much of a good thing."

The area around San Marcos has particularly tall cliffs abutting the lake and in recent years has become renowned for cliff diving.

The area supports extensive coffee and avocado orchards and a variety of farm crops, most notably corn and onions. Significant agricultural crops include: corn, onions, beans, squash, tomatoes, cucumbers, garlic, chile verde, strawberries and pitahaya fruit. The lake itself is a significant food source for the largely indigenous population.

The first volcanic activity in the region occurred about 11 million years ago, and since then the region has seen four separate episodes of volcanic growth and caldera collapse, the most recent of which began about 1.8 million years ago and culminated in the formation of the present caldera. The lake now fills a large part of the caldera, reaching depths of up to 340 m (1,120 ft).

The caldera-forming eruption is known as Los Chocoyos eruption and ejected up to 300 km³ (72 cu mi) of tephra. The enormous eruption dispersed ash over an area of some 6,000,000 square kilometres (2,300,000 sq mi): it has been detected from Florida to Ecuador, and can be used as a stratigraphic marker in both the Pacific and Atlantic oceans (known as Y-8 ash in marine deposits). A chocoyo is a type of bird which is often found nesting in the relatively soft ash layer.