Cnut (; Old English: Cnut cyning; Old Norse: Knútr inn ríki [ˈknuːtr ˈɪnː ˈriːkʲɪ]; died 12 November 1035), also known as Cnut the Great and Canute, was King of England from 1016, King of Denmark from 1018, and King of Norway from 1028 until his death in 1035. The three kingdoms united under Cnut's rule are referred to together as the North Sea Empire.

As a Danish prince, Cnut won the throne of England in 1016 in the wake of centuries of Viking activity in northwestern Europe. His later accession to the Danish throne in 1018 brought the crowns of England and Denmark together. Cnut sought to keep this power-base by uniting Danes and English under cultural bonds of wealth and custom. After a decade of conflict with opponents in Scandinavia, Cnut claimed the crown of Norway in Trondheim in 1028. The Swedish city Sigtuna was held by Cnut (he had coins struck there that called him king, but there is no narrative record of his occupation). In 1031, Malcolm II of Scotland also submitted to him, though Anglo-Norse influence over Scotland was weak and ultimately did not last by the time of Cnut's death.

Dominion of England lent the Danes an important link to the maritime zone between the islands of Great Britain and Ireland, where Cnut, like his father before him, had a strong interest and wielded much influence among the Norse–Gaels. Cnut's possession of England's dioceses and the continental Diocese of Denmark—with a claim laid upon it by the Holy Roman Empire's Archdiocese of Hamburg-Bremen—was a source of great prestige and leverage within the Catholic Church and among the magnates of Christendom (gaining notable concessions such as one on the price of the pallium of his bishops, though they still had to travel to obtain the pallium, as well as on the tolls his people had to pay on the way to Rome). After his 1026 victory against Norway and Sweden, and on his way back from Rome where he attended the coronation of the Holy Roman Emperor, Cnut deemed himself "King of all England and Denmark and the Norwegians and of some of the Swedes" in a letter written for the benefit of his subjects. The Anglo-Saxon kings used the title "king of the English". Cnut was ealles Engla landes cyning—"king of all England". Medieval historian Norman Cantor called him "the most effective king in Anglo-Saxon history".