Styrbjörn the Strong (Styrbjörn Sterki) or Styrbjörn the Swedish Champion (Styrbjörn svía kappi) (died c. 984) was, the son of the Swedish king Olof Björnsson, and the nephew of Olof's co-ruler and successor Eric the Victorious.
Styrbjörn was unusually big, strong and unruly, even for a Norseman, and and when he was only a young boy he managed to kill a courtier who accidentally had hit him on the nose with a drinking horn. When he was 12 years old he asked his uncle for his birthright, but when he was denied the co-rulership of Sweden he sulked for a long time on his father's mound.
When he was 16 the Thing decided that he was too unruly to be King of Sweden. As a compensation his uncle Eric gave him 60 well-equipped longships whereupon the frustrated Styrbjörn took his sister Gyrid and left. He ravaged the shores of the Baltic Sea and when he was twenty, he conquered the stronghold of Jomsborg from its founder Palnatoke, and became the ruler of the Jomsvikings.
After some time he allied with the Danish king Harald Bluetooth and married his sister Gyrid to him. Styrbjörn married Harold's daughter Tyra, whom he was given by Harald for conquering Jomsborg. Styrbjörn had a son, Torkel Styrbjörnsson with Tyra. Torkel had a daughter named Gytha Thorkelsdóttir, who married Godwin, Earl of Wessex and became the mother of Harold II of England.
Harald gave him even more warriors and Styrbjörn sought to reclaim the throne of Sweden. He sailed with a huge force which included 200 Danish longships in addition to his own Jomsvikings. When they arrived at Fyris in Uppland he burnt the ships in order to force his men to fight to the end, but the Danish force, seeing this, turned back to Denmark.
Styrbjörn marched alone with his Jomsvikings to Gamla Uppsala. His uncle was, however, prepared and had sent for reinforcements in all directions. During the first two days, the battle was even. In the evening, Eric went to the statue of Odin at the Temple at Uppsala where he sacrificed. He promised Odin that if he won the battle, he would belong to Odin and arrive at Valhalla in ten years from then. On the third day, Eric threw his spear over the enemy and said "I sacrifice you all to Odin". Styrbjörn and his sworn men stayed, and died.