Duke Frederick VIII (Danish: Frederik Christian August af Slesvig-Holsten-Sønderborg-Augustenborg; German: Friedrich Herzog von Schleswig-Holstein-Sonderburg-Augustenburg) (July 6, 1829 – January 14, 1880) was the German pretender to the throne of Schleswig-Holstein from 1863, although in reality Prussia took overlordship and real administrative power.

He was the eldest son of Christian August II, Duke of Schleswig-Holstein-Sonderburg-Augustenburg and Countess Louise Sophie of Danneskiold-Samsøe. He was ethnically perhaps the most Danish Prince of the Danish Royal dynasty in his generation (at the time of Denmark's most recent succession crisis). His family belonged to the House of Oldenburg, the royal house that included all the medieval Scandinavian royal dynasties among its distant forebears - which it shared with his rivals and relatives, other claimants to the Danish throne. Both lines claim descent from the medieval Danish House of Estridsen via Christian I of Denmark's ancestress Richeza of Denmark, Lady of Werle, the daughter of Eric V of Denmark, but Frederick also descended from Eric V's son Christopher II of Denmark whom no heir or monarch of Denmark had been descended from since Christopher III of Denmark. Frederik's paternal grandfather happened to have both grandfathers who were "Royal" dukes from the Oldenburg dynasty. Frederick also differed from his rivals in his specific ancestry among the contemporary Danish high nobility. His mother was from an ancient Danish family (Danneskiold-Samsøe), and his paternal grandmother Louise Auguste of Denmark was its royal princess. His paternal grandfather Frederik Christian II, Duke of Augustenborg numbered two ladies of Danish high nobility as his grandmothers (Danneskiold-Samsøe and Reventlow), and one Danish Countess as paternal great-grandmother (Ahlefeldt-Langeland). Frederick's family had high hopes that in the then-rising era of nationalism, this ancestry would be viewed with favour when the legal question over whose claim was strongest would be decided. The family groomed Frederick to become a King of Denmark.

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