Christian IV (12 April 1577 – 28 February 1648) was King of Denmark and Norway and Duke of Holstein and Schleswig from 1588 to 1648. His reign of 59 years, 330 days is the longest of Danish monarchs, and of all Scandinavian monarchies.

A member of the House of Oldenburg, Christian began his personal rule of Denmark in 1596 at the age of 19. He is remembered as one of the most popular, ambitious, and proactive Danish kings, having initiated many reforms and projects. Christian IV obtained for his kingdom a level of stability and wealth that was virtually unmatched elsewhere in Europe. He engaged Denmark in numerous wars, most notably the Thirty Years' War (1618–48), which devastated much of Germany, undermined the Danish economy, and cost Denmark some of its conquered territories. He rebuilt and renamed the Norwegian capital Oslo as Christiania after himself, a name used until 1925.