Lake Onega (also known as Onego, Onezhskoe ozero (from Russian, Онежское озеро), and Onezhskoe lake, Finnish: Ääninen or Äänisjärvi) is a lake in the Russian Federation. Its surface is 9,894 km² and its volume is 280 km³. It is the second largest lake in Europe.

Petrozavodsk, the capital of the Republic of Karelia, is situated on the Western shore of Lake Onega. The republic surrounds Lake Onega in West, North and East. In the South it borders the Leningrad Oblast and Vologda Oblast of Russia.

The UNESCO World Heritage site of Kizhi pogost lies on one of the islands in the Kizhi archipelago in the Northern part of the lake. Two magnificent 18th century churches are the centerpieces of this open-air museum of Northern Russian wooden architecture.

River Svir, that marks the southern boundary of Karelia, runs from the southern shore of Lake Onega to Lake Ladoga, and continues in the Neva to the Gulf of Finland.