Harald Fairhair (Old Norse: Haraldr Hárfagri; c. 850 – c. 932) was a Norwegian king. According to traditions current in Norway and Iceland in the eleventh and twelfth centuries, he reigned from c. 872 to 930 and was the first King of Norway. Supposedly, two of his sons, Eric Bloodaxe and Haakon the Good, succeeded Harald to become kings after his death.

Much of Harald's biography is uncertain. A couple of praise poems by his court poet Þorbjörn Hornklofi survive in fragments, but the extant accounts of his life come from sagas set down in writing around three centuries after his lifetime. His life is described in several of the Kings' sagas, none of them older than the twelfth century. Their accounts of Harald and his life differ on many points, but it is clear that in the twelfth and thirteenth centuries Harald was regarded as having unified Norway into one kingdom.

Since the nineteenth century, when Norway was in a personal union with Sweden, Harald has become a national icon of Norway and a symbol of independence. Though the king's sagas and medieval accounts have been critically scrutinised during the twentieth and early twenty-first centuries, Harald maintains a reputation as the father of the Norwegian nation. At the turn of the 21st century, a few historians have tried to argue that Harald Fairhair did not exist as a historical figure.

Old Norse hár translates straightforwardly into English as 'hair', but fagr, the adjective of which fagri is a form, is trickier to render, since it means 'fair, fine, beautiful' (but without the moral associations of English fair, as opposed to unfair). Although it is convenient and conventional to render hárfagri in English as 'fair-hair(ed)', in English 'fair-haired' means 'blond', whereas the Old Norse fairly clearly means 'beautiful-haired' (in contrast to the epithet which, according to some sources, Haraldr previously bore: lúfa, '(thick) matted hair'). Accordingly, some translators prefer to render hárfagri as 'the fine-haired' or 'fine-hair' (which, however, unhelpfully implies that Haraldr's hairs were thin) or even 'handsome-hair'.

Through the nineteenth and most of the twentieth centuries, historians broadly accepted the account of Harald Fairhair given by later Icelandic sagas. However, Peter Sawyer began to cast doubt on this in 1976, and the decades around 2000 saw a wave of revisionist research that suggested that Harald Fairhair did not exist, or at least not in a way resembling his appearance in sagas. The key arguments for this are as follows:

Scholarly consensus on Harald's historicity now falls into two camps. One suggests that the medieval Icelandic and Norwegian historiography of Harald Fairhair is part of an origin myth created to explain the settlement of Iceland, perhaps in which a cognomen of Haraldr Sigurðarson was transferred to a fictitious early king of all Norway. Sverrir Jakobsson has suggested that the idea of Iceland being settled by people fleeing an overbearing Norwegian monarch actually reflects the anxieties of Iceland in the early thirteenth century, when the island was indeed coming under Norwegian dominance. He has also suggested that the legend of Harald Fairhair developed in the twelfth century to enable Norwegian kings, who were then promoting the idea of primogeniture over the older custom of agnatic succession, to claim that their ancestors had had a right to Norway by lineal descent from the country's supposed first king.

One possibility advanced is that Harald Fairhair was based on a historical king called Harald, perhaps also known as "hárfagri", who ruled Vestlandet. The legend of this Harald later grew into the figure of medieval tradition. Historians who accept the early dating of skaldic poetry such as Claus Krag and Hans Jacob Orning tend to accept Harald's existence, while remaining skeptical regarding the saga accounts. In 2015, Hans Jacob Orning, building on then-recent archaeology and Krag's work, argued that Harald was based in Sogn, an area which the medieval Icelandic historian Snorri Sturluson associated with Harald, and which was a centre of power in the ninth century. In the skaldic poetry (which is generally considered authentic ninth-century work by linguists) the estates mentioned match a convenient network of estates with about a day's traveling distance between them, which would be ideal for a king ruling in Vestlandet, but not all of Norway. This reading could be consistent with the Historia Norwegiæ's account. While it is possible that Harald could have controlled other areas through jarls and client kings, this is difficult to prove with available archeology. Krag has noted that Snorri's account of Harald's origin in Vestfold might have been propaganda as the area of Viken was disputed between the Norwegian and Danish crown in the thirteenth century. Krag points of that Othere describes Viken as Danish territory and Hrafnsmál's description of the battle of Hafrsfjord suggest that Harald was attacked by "eastern" enemies that were routed and fled back east. He proposes that the battle was not part of a war of conquest but Harald defending his own territory from invaders. This idea offers a very different reading of the poem where its references to the dróttin Norðmanna (lord of the northmen) might have originally meant referred to the leader of the Norwegians in the battle, but later recontextualised as the lord of all Norwegians.

Harald is mentioned in several sagas, some which quotes supposedly older skaldic poetry. If the linguistic dating of the poems are correct, they represent the earliest accounts of Harald Fairhair.