Constantine II (Greek: Κωνσταντίνος Βʹ, Konstantínos II, pronounced [ˌkonstaˈdinos ðefˈteros]; born 2 June 1940) is the former King of Greece, reigning from 6 March 1964 until the abolition of the Greek monarchy on 1 June 1973.

Constantine is the only son of King Paul and Queen Frederica of Greece. As his family was forced into exile during the Second World War, he spent the first years of his childhood in Egypt and South Africa. He returned to Greece with his family in 1946 during the Greek Civil War. King George II died in 1947, and Constantine's father became the new king, making Constantine the crown prince.

He acceded as king in 1964 following the death of his father, King Paul. Later that year he married Princess Anne-Marie of Denmark with whom he eventually had five children. Although the accession of the young monarch was initially regarded auspiciously, his reign saw political instability that culminated in the Colonels' Coup of 21 April 1967. The coup left Constantine, as the head of state, little room to manoeuvre since he had no loyal military forces on which to rely. As a result, he reluctantly agreed to inaugurate the junta on the condition that it be made up largely of civilian ministers. On 13 December 1967, Constantine was forced to flee the country, following an unsuccessful countercoup against the junta. He remained (formally) the head of state in exile until the junta abolished the monarchy on 1 June 1973. The 1973 Greek republic referendum on 29 July, ratified the abolition.

There were questions concerning the validity of this referendum and whether people were pressured to vote for a republic. Therefore a fresh referendum was held after the restoration of democracy in 1974.

This second referendum was held after the fall of the junta as the 1974 Greek republic referendum on 8 December 1974 and confirmed the abolition of the monarchy and the establishment of the Third Hellenic Republic. Constantine, who was not allowed to return to Greece to campaign, accepted the results of the plebiscite.Constantine is also a former competitive sailor and Olympian, winning a gold medal in the 1960 Rome Olympics in the Dragon class, and later serving on the International Olympic Committee. Along with his fellow crew members of the Nireus sailing vessel, he was named one of the 1960 Greek Athletes of the Year.