Sinfiotli (or Sinfjotli), in Norse mythology, the son of Sigmund and his sister Signy, born to avenge the murder of their father Volsung and their brothers by King Siggeir. Signy, in the Scandinavian 'Volsunga Saga', was the only daughter of the hero Volsung, a descendant of the principal Norse god, Odin; Sigmund was the youngest of Volsung's ten sons. Signy was married against her will to Siggeir, king of the Goths, who later ambushed her father and brothers and tied them to a tree in the forest. She was unable to help them as a wild beast killed them, one at a time. Sigmund was the last one left alive. Signy was closely watched by Siggeir, but she was able to tell one of her servants to go to the woods and cover Sigmund's face with honey. The beast that came to attack him was attracted by the sweet odor and licked Sigmund's face, and he was able to catch the beast's tongue between his teeth and struggle free. Signy and Sigmund vowed to take revenge on Siggeir.
Believing that only a pure-blooded Volsung could aid them in avenging the Volsungs, Signy disguised herself as a gypsy and seduced her brother in his hut. Sinfiotli was conceived. Sigmund did not realize the gypsy was his sister. When Sinfiotli reached maturity, Signy sent him to Sigmund to be trained.
Sigmund and Sinfiotli together tried to kill Siggeir, but he captured them. With Signy's help, they escaped. They killed Siggeir and his men, setting his palace on fire. Signy refused to be saved, however; she revealed to Sigmund that Sinfiotli was his son by her, and then she stepped back into the burning palace.
Sigmund became king of Frankland, and Sinfiotli was his oldest son. Sigmund married Borghild of Denmark, by whom he had Helgi and Hamund. Later he had the hero Sigurd by his second wife, Hiordis. The 'Prose (or Younger) Edda' says that Sigmund was so tough that he could drink poison and not be harmed, while his sons Sinfiotli and Sigurd had such hard skins that poison did not harm them if it got on their bare flesh.
While Sigmund was married to Borghild, Sinfiotli slew her brother because they had wooed the same woman. At the next feast, Borghild gave ale to everyone but Sinfiotli. To him, she gave a drinking horn full of poison. Sinfiotli looked into the horn, saw the poison, and complained to his father. Since Sigmund was able to withstand poison internally, he merely took the horn and drank its contents himself. Borghild then brought Sinfiotli another horn full of poison, which Sigmund also drank. The third time she handed Sinfiotli the drinking horn she shamed him because he would not drink it. Sinfiotli complained as before to his father. Sigmund said: "Let your beard filter it, my son!" Sinfiotli, able to withstand poison only on his skin, drank the draught and fell down dead.
Sigmund carried Sinfiotli a long way in his arms to a long, narrow firth. There lay a small boat, and in it stood Odin, disguised as a ferryman. When Sigmund had put Sinfiotli's body in the boat, the ferryman shoved off from the land and the boat vanished, so the high god himself accompanied Sinfiotli on his journey to the realm of the dead.