The Lena (Russian: Ле́на, IPA: [ˈlʲenə]; Sakha: Өлүөнэ, Ölüöne) is the easternmost of the three great Siberian rivers that flow into the Arctic Ocean (the other two being the Ob River and the Yenisei River). It is the 11th longest river in the world and has the 9th largest watershed. It is the greatest river whose watershed is entirely within national ranges of Russia.

Rising at the height of 1,640 metres (5,381 ft) at its source Baikal Mountains south of the Central Siberian Plateau, 7 kilometres (4 mi) west of Lake Baikal, the Lena flows northeast, being joined by the Kirenga River, Vitim River and Olyokma River. From Yakutsk it enters the lowlands and flows north until joined by its right-hand affluent the Aldan River. The Verkhoyansk Range deflects it to the north-west; then, after receiving its most important left-hand tributary, the Vilyuy River, it makes its way nearly due north to the Laptev Sea, a division of the Arctic Ocean, emptying south-west of the New Siberian Islands by the Lena Delta - 30,000 square kilometres (11,583 sq mi) in area, and traversed by seven principal branches, the most important being Bykov, farthest east.