Harthacnut or Canute III (Danish: Hardeknud; "Tough-knot" c.1018 – 8 June 1042) is King of Denmark from 1035 to 1042 and King of England from 1040 to 1042.

He is the son of King Cnut the Great (who rules Denmark, Norway, and England) and Emma of Normandy.

When Cnut dies in 1035, Harthacnut struggles to retain his father's possessions.

Magnus I takes of control of Norway, but Harthacnut succeeds as King of Denmark and becomes King of England in 1040 after the death of his half-brother Harold Harefoot.

Harthacnut dies suddenly in 1042 and is succeeded by Magnus in Denmark and Edward the Confessor in England.

Harthacnut is the last Danish king to rule England.