Which lake is the largest entirely within Russia?

The largest lake entirely within Russia is Lake Baikal.

Lake Baikal lies in southern Siberia, between Irkutsk Oblast and the Republic of Buryatia. It is a freshwater rift lake formed where the Earth’s crust is slowly pulling apart. At about 25–30 million years old, it is also regarded as the world’s oldest existing lake.

Baikal is not the world’s largest lake by surface area—the Caspian Sea is larger—but its exceptional depth gives it the greatest volume of fresh water of any lake. Its deepest point reaches 1,642 metres, and the lake contains roughly 22–23% of the world’s unfrozen surface fresh water, more than all the North American Great Lakes combined.

The wording “entirely within Russia” matters. Lake Baikal is wholly in Russia, while several other huge lakes cross national borders or are classified differently. Lake Ladoga is Russia’s largest lake by surface area in Europe, but it is much smaller than Baikal. Baikal’s isolated ecosystem includes many endemic species, including the Baikal seal.