Jarl Sigvaldi was the son of the Scanian Jarl Strut-Harald and the brother of Þorke(ti)ll inn hávi. He succeeded Palnatoke as the chieftain of the Jomsvikings, but he proved more wise than brave.
In order to win Astrid, the daughter of the Wendish chieftain Burislav, he promised to liberate the Wends of the tribute they had to pay to the Danes. He fulfilled his promise by sailing to Zealand where he sent the message to Sveinn Forkbeard that he had important tidings, but had fallen ill and could not come in person to bring them to him. As Sweyn was curious, he went aboard Sigvaldi's ship and was captured by the Jomsvikings. To be liberated the Danish king had to grant independence to both the Jomsvikings and to the Wends, in addition to paying a king's ransom.
At his father Strut-Harald's funeral, Sigvaldi was advised by Sveinn to attack Norway and to depose Haakon Jarl. This promise led to the Battle of Hjörungavágr in 986, from which Sigvald fled with disgrace.
In 1000, Sigvaldi proved to be treacherous at the Battle of Swold, by luring Óláfr Tryggvason to the battle and by deserting him in the heat of battle.