Siggeir is a mythological figure in the Vǫlsunga saga. He was the king of Gautland (Swedish province of Götaland).
According to the Vǫlsunga saga, Siggeir married Signý, the sister of Sigmundr and the daughter of king Vǫlsungr. At the banquet Óðinn appears in disguise wearing a cape and a hood and sticks a sword in the tree Barnstokkr. Then he said that whoever managed to pull the sword out could keep it. Siggeir and everyone else tried but only Sigmundr succeeded. Siggeir generously offered three times the sword's value, but Sigmundr mockingly refused. Siggeir was offended and went home the next day thinking of revenge.
Siggeir invites Sigmundr, his father Vǫlsungr and Sigmundr's nine brothers to a visit in Gautland to see the newlyweds three months later. When the Vǫlsungar had arrived they were attacked by the Gauts and king Vǫlsungr was killed and his sons captured.
Signý beseeched her husband to spare her brothers and to put them in stocks instead of killing them. As Siggeir thought that the brothers deserved to be tortured before they were killed, he agreed.
He then let his shape-shifting mother turn into a wolf and each night devour one of the brothers, until only Sigmundr remained. Signý had a servant smear honey on the face of Sigmundr and when the she-wolf arrived she started licking the honey off Sigmundr's face. As she licked, she stuck her tongue into Sigmundr's mouth, whereupon Sigmundr bit her tongue off, killing her. Sigmundr then hid in the forests of Gautland and Signý brought him everything he needed.
Signý gave Siggeir two sons and when the oldest one was ten years old, she sent him to Sigmundr to train him to avenge the Vǫlsungar. The boy did not stand a test of courage so Signý asked Sigmundr to kill her worthless son. The same thing happened to Siggeir's second son.
Signý came to Sigmundr in the guise of a vǫlva and she and her brother committed incest and had the son Sinfjǫtli. After some adventures Siggeir is killed by Sigmundr and Sinfjǫtli.