Dag the Wise or Dagr spaki was a mythological Swedish king of the House of Ynglings. He was the son of Dyggvi, the former king. According to legend, he could understand the speech of birds and had a sparrow that gathered news for him from many lands. When the bird was killed on one of these trips, Dag invaded Reidgotaland, in order to avenge it. There he was ambushed by a thrall and killed.

The Ynglings were a dynasty of kings, first in Sweden and later in Norway, primarily attested through the poem Ynglingatal. The dynasty also appears as Scylfings in Beowulf. When Beowulf and Ynglingatal were composed sometime in the eighth to tenth centuries, the respective scop and skald (poet) expected his audience to have a great deal of background information about these kings, which is shown in the allusiveness of the references.