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

His mother was from an ancient Danish family (Danneskiold-Samsøe), and his paternal grandmother Louise Auguste of Denmark was its royal princess.

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.

Young Frederick's father found himself in an untenable position after the collapse of Prussian support and defeat of his own government at the end of the First Schleswig War in 1851.

He renounced his claims as first in line to inherit the twin duchies in favour of the king of Denmark and his successors on 31 March 1852 in return for a financial compensation.

Frederick's marriage in 1856 was part of an appeal to German nationalism (however, his younger brother married a daughter of Queen Victoria).

She was the second daughter of Ernst Christian Carl IV, Duke of Hohenlohe-Langenburg and Princess Feodora of Leiningen, elder half-sister of Queen Victoria.