In Norse mythology, Dagr (Old Norse "day") is day personified. This personification appears 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. In both sources, Dagr is stated to be the son of the god Dellingr and is associated with the bright-maned horse Skinfaxi, who "draw[s] day to mankind". Depending on manuscript variation, the Prose Edda adds that Dagr is either Dellingr's son by Nótt, the personified night, or Jörð, the personified Earth. Otherwise, Dagr appears as a common noun simply meaning "day" throughout Old Norse works. Connections have been proposed between Dagr and other similarly named figures in Germanic mythology.

Dagr is mentioned in stanzas 12 and 25 of the poem Vafþrúðnismál. In stanza 24, the god Odin (disguised as "Gagnráðr") asks the jötunn Vafþrúðnir from where the day comes, and the night and its tides. In stanza 25, Vafþrúðnir responds:

In Sigrdrífumál, after the valkyrie Sigrdrífa is woken from her sleep curse by the hero Sigurd, Sigurd asks her name, and she gives him a "memory-drink" of a drinking horn full of mead, and then Sigrdrifa says a prayer. The first verse of this prayer features a reference to the "sons of Dagr" and the "daughter of Nótt":