In Norse mythology, Mímisbrunnr (Old Norse "Mímir's well") is a well associated with the being Mímir, located beneath the world tree Yggdrasil. Mímisbrunnr is attested in the Poetic Edda, compiled in the 13th century from earlier traditional sources, and the Prose Edda, written in the 13th century by Snorri Sturluson. Both sources relate that the god Odin once placed one of his eyes within the well. The Prose Edda details that well is located beneath one of three roots of the world tree Yggdrasil, a root that passes into the land of the frost jötnar where the primordial plane of Ginnungagap once existed. In addition, the Prose Edda relates that the water of the well contains much wisdom, and that Odin's eye sacrifice to the well was in exchange for a drink from it.

In the Poetic Edda poem Völuspá, a völva recounts to Odin that she knows that Odin once placed one of his eyes in Mímisbrunnr as a pledge, and that Mímir drinks from the well every morning:

The above stanza is absent from the Hauksbók manuscript version of the poem. Elsewhere in the poem, the völva mentions a scenario involving the hearing or horn (depending on translation of the Old Norse noun hljóð—bolded for purpose of illustrated) of the god Heimdallr: