Lake Baikal (, Russian: Oзеро Байкал, romanized: Ozero Baykal [ˈozʲɪrə bɐjˈkaɫ]) is a rift lake in Russia. It is situated in southern Siberia, between the federal subjects of Irkutsk Oblast to the northwest and the Republic of Buryatia to the southeast. With 23,615.39 km³ (5,670 cu mi) of water, Lake Baikal is the world's largest freshwater lake by volume, containing 22–23% of the world's fresh surface water, more than all of the North American Great Lakes combined. It is also the world's deepest lake, with a maximum depth of 1,642 metres (5,387 feet; 898 fathoms), and the world's oldest lake, at 25–30 million years. At 31,722 km² (12,248 sq mi)—slightly larger than Belgium—Lake Baikal is the world's seventh-largest lake by surface area. It is among the world's clearest lakes.

Lake Baikal is home to thousands of species of plants and animals, many of them endemic to the region. It is also home to Buryat tribes, who raise goats, camels, cattle, sheep, and horses on the eastern side of the lake, where the mean temperature varies from a winter minimum of −19 °C (−2 °F) to a summer maximum of 14 °C (57 °F). The region to the east of Lake Baikal is referred to as Transbaikalia or as the Transbaikal, and the loosely defined region around the lake itself is sometimes known as Baikalia. UNESCO declared Baikal a World Heritage Site in 1996.