In Greek mythology, Lethe (; Ancient Greek: Λήθη Lḗthē; Ancient Greek: [lɛ̌ːtʰɛː], Modern Greek: [ˈliθi]) was one of the rivers of the underworld of Hades. In Classical Greek, the word lethe (λήθη) literally means "forgetting", "forgetfulness". The river is also known as Amelēs Potamos, or the "river of unmindfulness".

The Lethe flowed around the cave of Hypnos and through the Underworld where all those who drank from it experienced complete forgetfulness. The river was often associated with Lethe, the personification of forgetfulness and oblivion, who was the daughter of Eris (Strife).