Harald III was an illegitimate son of Danish king Sweyn II Estridsson, and contested the crown with some of his brothers.

Harald was married to his cousin Margareta Hasbjörnsdatter, but did not leave any heirs, and was succeeded by his brother Canute IV the Saint.

[3] After the death of his father in 1076, Harald was elected king in competition with his younger brother, Canute, at an assembly at Isøre near Odsherred.

[1] During his rule, Harald met opposition from a number of his brothers,[1] likely including Canute,[2] who enlisted the support of Olaf III of Norway.

Pope Gregory VII mediated, advising Olaf not to take sides, and for Harald to share power with his brothers.