Harthacnut or Cnut I was a semi-legendary King of Denmark. The old Norse story Ragnarssona þáttr makes Harthacnut son of the semi-mythic viking chieftain Sigurd Snake-in-the-Eye, himself one of the sons of the legendary Ragnar Lodbrok. The saga in turn makes Harthacnut the father of the historical king, Gorm. It has been suggested he is to be identified with the Hardegon of Northmannia whose early-10th century conquest of Denmark was related by Adam of Bremen.

The monarchy of Denmark is a constitutional institution and a historic office of the Kingdom of Denmark. The Kingdom includes Denmark proper and the autonomous territories of the Faroe Islands and Greenland. The Kingdom of Denmark was already consolidated in the 8th century, whose rulers are consistently referred to in Frankish sources as "kings". Under the rule of King Gudfred in 804 the Kingdom may have included all the major provinces of medieval Denmark.