Lake Ilmen (Russian: Озеро Ильмень, Finnish: Ilmajärvi), a lake in Novgorod Oblast of Russia.

Ilmen is about 45 km long ang 35 km wide and its surface area is 982 sq km (may vary from 733 to 2090 sq km. depending on water level). The lake is fed by some 50 inflowing rivers, the four main ones being Msta , Pola, Lovat' , Shelon' and is drained through a single outlet, the Volkhov. The city of Novgorod lies 6 km below the lake's outflow.

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