Harald III Sigurdsson (born 1015, Norway—died Sept. 25, 1066, Stamford Bridge, Yorkshire, England.) was king of Norway from 1045 to his death in 1066. He was the son of Sigurd Sow (Syr), a chieftain in eastern Norway, and of Estrid, mother of the Norwegian king Olaf II Haraldsson (St. Olaf).

In 1030, at the age of 15, Harald fought alongside Olaf II Haraldsson in the Battle of Stiklestad in which Olaf was killed. Harald then fled to Kievan Rus’, where he served under the grand prince of Kiev, Yaroslav I the Wise. Harald later married Yaroslav’s daughter, Elizabeth.

After enlisting in the military service of the Byzantine emperor Michael IV who reigned 1034–41, he fought with the imperial armies in Sicily and Bulgaria and, according to Haralds saga Sigurðarsonar, made a pilgrimage to Jerusalem. Harald’s military exploits under Michael IV appear in both Byzantine and Norse medieval sources.

In 1045 Harald returned to Norway and agreed to share the Norwegian throne with the reigning king, his nephew Magnus I Olafsson. Harald became the sole ruler of Norway in 1047 when Magnus died during a military expedition against Denmark.

For the next fifteen years Harald attempted to win the Danish throne from Sweyn II. In 1062, after Sweyn was defeated in the Battle of Niså, the two rulers recognized each other as sovereign in their own countries.

Harald expanded the Norwegian possessions in the Orkney, Shetland, and Hebrides. In 1066, Harald joined forces with Earl Tostig in an attempt to win the English throne from Harold Godwinson (Harols II). Although the invading forces were initially successful in the Battle of Fulford, they were defeated in the Battle of Stamford Bridge, where Harald Hardrada was killed.