Lake Ilmen (Russian: И́льмень, IPA: [ˈilʲmʲɪnʲ]) is a large lake in Novgorod Oblast, Russia. A historically important lake, it formed a vital part of the medieval trade route from the Varangians to the Greeks. The city of Veliky Novgorod, which is a major trade center of the route, lies six kilometres (3.7 mi) below the lake's sole outflow, via the Volkhov River.
According to Max Vasmer's Etymological Dictionary, the name of the lake originates from Finnic Ilmajärvi, which means "air lake". Thanks to Novgorodian colonisation, many lakes in Russia have names deriving from Lake Ilmen. Yuri Otkupshchikov has argued that the presence of the name "Ilmen" in Southern Russia cannot be explained by Novgorodian colonisation alone, and proposed a Slavic etymology instead.