Christian IX (8 April 1818 – 29 January 1906) was King of Denmark from 1863 until his death in 1906. From 1863 to 1864, he was concurrently Duke of Schleswig, Holstein and Lauenburg.

A younger son of Frederick William, Duke of Schleswig-Holstein-Sonderburg-Glücksburg, Christian grew up in the Duchy of Schleswig as a prince of Schleswig-Holstein-Sonderburg-Glücksburg, a junior branch of the House of Oldenburg which had ruled Denmark since 1448. Although having close family ties to the Danish royal family, he was originally not in the immediate line of succession to the Danish throne. Following the early death of the father in 1831, Christian grew up in Denmark and was educated at the Military Academy of Copenhagen. After unsuccessfully seeking the hand of Queen Victoria of the United Kingdom in marriage, he married his double second cousin, Princess Louise of Hesse-Kassel, in 1842.

In 1852, Christian was chosen as heir-presumptive to the Danish throne in light of the expected extinction of the senior line of the House of Oldenburg. Upon the death of King Frederick VII of Denmark in 1863, Christian (who was Frederick's second cousin and husband of Frederick's paternal first cousin, Louise of Hesse-Kassel) acceded to the throne as the first Danish monarch of the House of Glücksburg.The beginning of his reign was marked by the Danish defeat in the Second Schleswig War and the subsequent loss of the duchies of Schleswig, Holstein and Lauenburg which made the king immensely unpopular. The following years of his reign were dominated by political disputes as Denmark had only become a constitutional monarchy in 1849 and the balance of power between the sovereign and parliament was still in dispute. In spite of his initial unpopularity and the many years of political strife, where the king was in conflict with large parts of the population, his popularity recovered towards the end of his reign, and he became a national icon due to the length of his reign and the high standards of personal morality with which he was identified.

Christian's six children with Louise married into other European royal families, earning him the sobriquet "the father-in-law of Europe". Among his descendants are Queen Margrethe II of Denmark, King Philippe of Belgium, King Harald V of Norway, Grand Duke Henri of Luxembourg, Queen Elizabeth II of the United Kingdom, former King Constantine II of Greece, and King Felipe VI of Spain.