In Greek mythology, Lethe (; Ancient Greek: Λήθη Lḗthē; Ancient Greek: [lɛ̌ːtʰɛː], Modern Greek: [ˈliθi]), also referred to as Lemosyne, was one of the five rivers of the underworld of Hades. Also known as the Ameles potamos (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. Lethe was also the name of the Greek spirit of forgetfulness and oblivion, with whom the river was often identified.

In Classical Greek, the word lethe (λήθη) literally means "oblivion", "forgetfulness", or "concealment". It is related to the Greek word for "truth", aletheia (ἀλήθεια), which through the privative alpha literally means "un-forgetfulness" or "un-concealment".