river in Mongolia and east-central Russia. It is formed by the confluence of the Ider and Delger rivers. It is Mongolia's principal river and is the most substantial source of water for Lake Baikal (Baikal, Lake).

The Delger rises in the Sangilen Mountains on the border between Mongolia and the Russian republic of Tuva, and the headwaters of the Ider lie in the Hangayn Mountains in Mongolia to the south. From their confluence the Selenga flows northeast to receive the Orhon River at Sühbaatar. It continues north into Russia and bends eastward to Ulan-Ude, capital of Buryatia. From there the river flows north to Tataurovo, where it swings westward and then flows through a delta into Lake Baikal. From Sühbaatar to its mouth the Selenga flows down a gentle slope, giving Mongolia access to the plains of Siberia (Asiatic Russia). The Trans-Siberian Railroad follows the river's valley upstream to Ulan-Ude, where a branch continues to Sühbaatar, and then up the Orhon to Ulaanbaatar, the capital of Mongolia.

The Selenga is 920 miles (1,480 km) long and is navigable from its mouth to beyond Sühbaatar in May–October, when it is ice-free. The river basin has a subarctic climate, with severe winters and warm summers. The river freezes from November to April, and snowmelt increases the river's flow during the spring and early summer. Its 173,000-square-mile (448,000-square-km) basin includes the Mongolian tributaries the Orhon, Hanuy, and Egiyn and the Russian Chikoy, Khilok, Uda, Dzhida, and Temnik rivers.