Gunnar, in Norse mythology, son of King Giuki and Queen Grimhild of the Nibelungs, or Burgundians, brother of the beautiful Gudrun and the warrior Hogni. Assisted by an unwitting Sigurd, Gunnar won the warrior maiden Brunhild for his wife through trickery. He and Sigurd had sworn an oath of brotherhood. Gunnar later conspired in the murder of Sigurd, and was himself slain at the court of King Atli of the Huns. Brunhild, a powerful Valkyrie, was to marry only the most valiant warrior; she lay sleeping encircled by a magical ring of fire. Gunnar and his horse, Goti, were unable to traverse the flames, so Sigurd and his horse, Grani, exchanged forms and names with Gunnar and Goti, and rode through the flames without hesitation. Brunhild believed Gunnar was worthy of her hand, and she consented to marriage.
Upon her marriage, Brunhild lost her supernatural powers and unhappily resigned herself to her union with Gunnar. When she discovered how she had been deceived, however, her smoldering resentment became outright hatred directed against Sigurd, the only warrior truly worthy of her hand. In one version of the myth, she instigated Gunnar and Hogni against Sigurd; in another variation, Gunnar himself was so jealous of Sigurd that he participated in the plot to kill his oath-brother.
After Sigurd's murder, Brunhild killed herself. Sigurd's widow, Gudrun, married King Atli of the Huns. Atli demanded that Gunnar and Hogni relinquish the Nibelung treasure, which they had hid. The brothers refused to give up the treasure, which Sigurd had won from the dragon Fafnir. In retaliation, Atli had Hogni's heart cut out, and then had Gunnar thrown into a snake pit to die in agony. Gunnar attempted to charm the snakes with a harp, but one adder resisted the enchantment and fatally bit him. In the Germanic epic 'Song of the Nibelungs' (Nibelungenlied), Gunnar is called Gunther.