Christian IV (12 April 1577 – 28 February 1648) was King of Denmark and Norway and Duke of Holstein and Schleswig from 1588 until his death in 1648. His reign of 59 years and 330 days is the longest in Scandinavian history.

A member of the House of Oldenburg, Christian began his personal rule of Denmark-Norway in 1596 at the age of 19. He is remembered as one of the most popular, ambitious, and proactive Danish-Norwegian kings, having initiated many reforms and projects. Christian IV obtained for his kingdoms a level of stability and wealth that was virtually unmatched elsewhere in Europe. He engaged Denmark-Norway in numerous wars, most notably the Thirty Years' War (1618–1648), which devastated much of Germany, undermined the Danish economy, and cost Denmark-Norway some of its conquered territories. He rebuilt and renamed the Norwegian capital Oslo as Christiania after himself, a name used until 1925.

Christian was born at Frederiksborg Castle in Denmark on 12 April 1577 as the third child and eldest son of King Frederick II of Denmark–Norway and Sofie of Mecklenburg-Schwerin. He was descended, through his mother's side, from King John of Denmark, and was thus the first descendant of King John to assume the crown since the deposition of King Christian II.

At the time, Denmark was still an elective monarchy, so in spite of being the eldest son Christian was not automatically heir to the throne. But Norway was a hereditary monarchy, and electing someone else would result in the end of the union of the crowns. However, in 1580, at the age of 3, his father had him elected Prince and successor to the throne of Denmark.

At the death of his father on 4 April 1588, Christian was just 10 years old. He succeeded to the throne, but as he was still under-age a regency council was set up to serve as the trustees of the royal power while Christian was still growing up. It was led by chancellor Niels Kaas (1535–1594) and consisted of the Rigsraadet council members Peder Munk (1534–1623), Jørgen Ottesen Rosenkrantz (1523–1596) and Christoffer Valkendorff (1525–1601). His mother Queen Dowager Sophie, 30 years old, had wished to play a role in the government, but was denied by the council. At the death of Niels Kaas in 1594, Jørgen Rosenkrantz took over leadership of the regency council.

Christian continued his studies at Sorø Academy where he had a reputation as a headstrong and talented student.

In 1595, the Council of the Realm decided that Christian would soon be old enough to assume personal control of the reins of government. On 17 August 1596, at the age of 19, Christian signed his haandfæstning (lit. "Handbinding" viz. curtailment of the monarch's power, a Danish parallel to Magna Carta), which was an identical copy of his father's from 1559.

Twelve days later, on 29 August 1596, Christian IV was crowned at the Church of Our Lady in Copenhagen by the Bishop of Zealand, Peder Jensen Vinstrup (1549–1614). He was crowned with a new Danish Crown Regalia which had been made for him by Dirich Fyring (1580–1603), assisted by the Nuremberg goldsmith, Corvinius Saur.