Frederick III (Danish: Frederik III; March 18, 1609 – February 9, 1670) is king of Denmark and Norway from 1648 until his death, and institutes Denmark as an absolute monarchy in 1660.

He was born as the second-eldest son of Christian IV of Denmark and Anne Catherine of Brandenburg.

Frederick is only considered an heir to the throne after the death of his older brother Prince Christia n in 1647.

At the death of Christian IV, Frederick concedes significant influence to the nobility, in order to be elected king.

As king, he fights two wars against Sweden.

He is defeated in the 1657–1658 Dano-Swedish War, but attains great popularity when he weathers the 1659 Assault on Copenhagen and wins the 1658–1660 Dano-Swedish War.

Later this year, Frederick uses his popularity to disband the elective monarchy in favor of absolute monarchy, which is to last until 1848.

He marries Sophie Amalie of Brunswick-Lüneburg, with whom he has a son, Christian, who succeeds him to the throne as Christian V of Denmark.