Jonakr (also spelled Ionakr), in Norse mythology, a king of Denmark, and in some versions of the saga of the Volsungs, the third husband of the beautiful Gudrun (Guthrun) of the Nibelungs. Gudrun had been the wife of the Volsung hero Sigurd, whom she had loved deeply. After Sigurd had been treacherously murdered, Gudrun married the avaricious King Atli of the Huns. Gudrun took vengeance on Sigurd's murderers and on Atli, and set fire to Atli's great hall. According to the 'Prose (or Younger) Edda', she then leapt into the sea to drown herself but drifted across the fjord and washed up alive on the shore of Denmark, ruled by King Jonakr.
Jonakr took Gudrun in and married her. Gudrun had a daughter by Sigurd named Swanhild (Svanhild), and Gudrun also had three sons by Jonakr, named Sorli, Hamdir, and Erp. These sons had hair as black as a raven. They all were killed in an attempt to avenge their lovely stepsister Swanhild, who died at the hands of her betrothed, the jealous King Jormunrek.