Frederick VIII, Duke of Schleswig-Holstein and of Schleswig-Holstein-Sonderburg-Augustenburg 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.

The House of Schleswig-Holstein-Sonderburg-Augustenburg was a branch of the dukes of Schleswig-Holstein-Sonderburg of the House of Oldenburg. The line descended from Alexander, Duke of Schleswig-Holstein-Sonderburg. Like all of the secondary lines from the Sonderburg branch, the heads of the House of Schleswig-Holstein-Sonderburg-Augustenburg were first known as Dukes of Schleswig-Holstein and Dukes of Sonderburg. The family took its name from its ancestral home, Augustenborg Palace in Augustenborg, Denmark.