Harald Sigurdsson (Old Norse: Haraldr Sigurðarson; c. 1015 – 25 September 1066), also known as Harald III of Norway and given the epithet Hardrada in the sagas, was King of Norway from 1046 to 1066. He unsuccessfully claimed the Danish throne until 1064 and the English throne in 1066. Before becoming king, Harald spent 15 years in exile as a mercenary and military commander in Kievan Rus' and chief of the Varangian Guard in the Byzantine Empire. In his chronicle, Adam of Bremen called him the "Thunderbolt of the North".

In 1030, the fifteen-year-old Harald fought in the Battle of Stiklestad alongside his half-brother Olaf Haraldsson. Olaf sought to reclaim the Norwegian throne, which he had lost to Danish king Cnut two years previously. Olaf and Harald were defeated by forces loyal to Cnut, and Harald was forced into exile to Kievan Rus'. Thereafter, he was in the army of Grand Prince Yaroslav the Wise, becoming captain, until he moved on to Constantinople with his companions around 1034. In Constantinople, he rose quickly to become the commander of the Byzantine Varangian Guard, seeing action on the Mediterranean Sea, in Asia Minor, Sicily, possibly in the Holy Land, Bulgaria and in Constantinople itself, where he became involved in the imperial dynastic disputes. Harald amassed wealth whilst in the Byzantine Empire, which he shipped to Yaroslav in Kievan Rus' for safekeeping. In 1042, he left the Byzantine Empire, returning to Kievan Rus' to prepare to reclaim the Norwegian throne. In his absence the Norwegian throne had been restored from the Danes to Olaf's illegitimate son Magnus the Good.

In 1046, Harald joined forces with Magnus's rival in Denmark, the pretender Sweyn II of Denmark, raiding the Danish coast. Magnus, unwilling to fight his uncle, agreed to share the kingship with Harald, since Harald in turn would share his wealth with him. The co-rule ended abruptly the next year as Magnus died: Harald became the sole ruler of Norway. Domestically, Harald crushed opposition, and outlined the unification of Norway. Harald's reign was one of relative peace and stability, and he instituted a coin economy and foreign trade. Seeking to restore Cnut's "North Sea Empire", Harald claimed the Danish throne, and spent nearly every year until 1064 raiding the Danish coast and fighting his former ally, Sweyn. Although the campaigns were successful, he was never able to conquer Denmark.

Not long after Harald had renounced his claim to Denmark, the former Earl of Northumbria, Tostig Godwinson, brother of English king Harold Godwinson, pledged his allegiance to Harald, inviting him to claim the English throne. Harald assented, invading northern England with 10,000 troops and 300 longships in September 1066, defeating the English regional forces of Northumbria and Mercia in the Battle of Fulford near York on 20 September. Harald was defeated and killed in a surprise attack by Harold Godwinson's forces in the Battle of Stamford Bridge on 25 September, which wiped out his army. Historians often consider Harald's death the end of the Viking Age.

Harald's most famous epithet is Old Norse harðráði, which has been translated variously as 'hard in counsel', 'tyrannical', 'tyrant', 'hard-ruler', 'ruthless', 'savage in counsel', 'tough', and 'severe'. While Judith Jesch has argued for 'severe' as the best translation, Alison Finlay and Anthony Faulkes prefer 'resolute'. Harðráði has traditionally been Anglicised as 'Hardrada', though Judith Jesch characterises this form as "a bastard Anglicisation of the original epithet in an oblique case". This epithet predominates in the later Icelandic saga-tradition.

However, in a number of independent sources associated with the British Isles, mostly earlier than the Icelandic sagas, Harald is given epithets deriving from Old Norse hárfagri (literally 'hair-beautiful'). These sources include:

In Icelandic sagas the name Harald Fairhair is more famously associated with an earlier Norwegian king, and twentieth-century historians assumed that the name was attached to Harald Hardrada in error by Insular historians. However, recognising the independence of some of the Insular sources, historians have since favoured the idea that Harald Hardrada was widely known as Harald Fairhair, and indeed now doubt that the earlier Harald Fairhair existed in any form resembling the later saga-accounts.

Sverrir Jakobsson has suggested that 'fairhair' "might be the name by which King Harald wished himself to be known. It must have been his opponents who gave him the epithet "severe" (ON. harðráði), by which he is generally known in thirteenth-century Old Norse kings' sagas".