The Selenga River (Selenge River, Mongolian: Сэлэнгэ мөрөн, Selenge mörön; Buryat: Сэлэнгэ гол, Selenge gol, Сэлэнгэ мүрэн, Selenge müren; Russian: Селенга́) is a major river in Mongolia and Buryatia, Russia. Its source rivers are the Ider River and the Delgermörön river. It flows into Lake Baikal and has a length of 992 kilometres (616 mi) or 1,024 kilometres (636 mi), according to other sources. The Selenga River is the headwaters of the Yenisei-Angara River system. Carrying 935 cubic metres per second (33,000 cu ft/s) of water into Lake Baikal, it comprises almost half of the riverine inflow and forms a wide delta of 680 square kilometres (260 sq mi), when it reaches the lake.
The name derives from Evenkisele 'iron' + -nga (suffix).Selenge Province in northern Mongolia is derived from the name of this river. The Mongolian verb "seleh" means to swim.
Railway bridge over the Selenga River outside of Ulan Ude, Russia.