Ilmen () is a historically important lake in the Novgorod Oblast of Russia, formerly a vital part of the Trade route from the Varangians to the Greeks. The city of Novgorod lies six kilometers below the lake's outflow.

Its average surface area is 982 km² (may vary from 733 to 2,090 km² depending on water level). The lake is fed by 52 inflowing rivers, the four main ones being Msta, Pola, Lovat, Shelon and is drained through a single outlet, the Volkhov, into the Lake Ladoga, and subsequently via the Neva into the Gulf of Finland.

The water level is regulated by the Volkhov hydroelectric dam situated downstream the Volkhov River. Water temperature in July is 19-20°C. Bathing season is about 90 days. Fishery. Shipping lines are Novgorod – Staraya Russa and Novgorod – Shimsk.

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