Ilmen, Freshwater lake in Novgorod Oblast, Russia.

Ilmen is a freshwater body in the center of the Novgorod lowlands, receiving water from roughly fifty rivers. The main tributaries include Msta, Pola, Lovat, Psizha and Shelon, which drain from the surrounding hills into the basin.

Lake Ilmen formed after the last ice age on the site of older water systems. Its basin lies on the East European Platform, where crystalline basement rock sits 600 to 2,000 metres down and was reached only by deep boreholes near Novgorod and Valday. Around 200 million years ago the sea finally withdrew from the area.

The name Ilmen became a common noun for small, reed-choked lakes formed from river floods or old channels. The chronicle Tale of Slovene and Rus and the city of Slovensk links it to their sister Ilmera. Scholars most often derive it from Finnic Ilma-järvi, "lake of (bad) weather" or "upper sky lake"; Y. V. Otkupshchikov instead proposes Slavic il plus -men, "silty lake".

Passenger boats run during summer for roughly ninety days on routes between Veliky Novgorod, Staraya Russa and Shimsk. Navigation depends on water level, which changes with rainfall and snowmelt in spring.

The surface shifts between 733 and 2,090 square kilometers (283 and 807 square miles) depending on inflow, meaning the shoreline sometimes moves several kilometers. This change creates temporary islands and wide shallow zones that vanish during wet years or appear in dry months.