Frederik IX of Denmark (March 11, 1899 – January 14, 1972) was King of Denmark from 1947 until 1972. He married HRH Princess Ingrid of Sweden on May 24, 1935. They had three children, Queen Margrethe II of Denmark, HRH Princess Benedikte of Denmark, who is married to HSH Prince Richard zu Sayn-Wittgenstein-Berleburg, and HM Queen Anne-Marie of Greece, who in 1964 married HM King Constantine II of Greece.

When he had no sons, in accordance with Denmark's Salic law, it was expected that his brother would inherit the throne. However, in 1953, an Act of Succession was passed, changing the method of succession from Salic law to male primogeniture, meaning that his eldest daughter Margrethe could succeed if he had no sons. He never did have any sons, so Margrethe did succeed.