Mímir (Old Norse: Mímir) is a primordial being of wisdom associated with the well of knowledge that bears his name — Mímisbrunnr — located beneath one of the three roots of the world-tree Yggdrasil, as described in Völuspá and Gylfaginning. Odin sacrificed one of his eyes at this well in exchange for a drink of its wisdom-giving waters. Mímir himself is described by Snorri as the wisest being in the world.

After the Æsir-Vanir War, Mímir was sent as a hostage to the Vanir along with Hœnir. The Vanir, feeling deceived because Hœnir relied entirely on Mímir’s counsel when separated from him, beheaded Mímir. Odin retrieved the head, preserved it with herbs, and reanimated it with incantation (galdr), allowing Mímir’s severed head to continue speaking and providing counsel — preserved in Ynglinga saga and referenced in Völuspá (st. 46), which states that Odin speaks with the head of Mímir before the onset of Ragnarök.

Mímir’s exact nature is ambiguous: he is given as a hostage from the Æsir’s side, but the primordial character of his wisdom and the location of his well suggest a being older than the current divine order.