Duke Frederick VIII (July 6, 1829 - January 14, 1880) (Friedrich Herzog von Schleswig-Holstein-Sonderburg-Augustenburg in German), was the father-in-law of Kaiser Wilhelm II of Germany.

Frederick became the symbol of the nationalist German independence-movement in Schleswig-Holstein, after his father in exchange for money had renounced his claims as first in line to inherit the twin-duchies of Schleswig and Holstein, following the London protocol of May 8th, 1852, which concluded the First war of Schleswig.

In November 1863 Frederick claimed the twin-duchies in succession after King Frederick VII of Denmark, who also was the Duke of Schleswig and Holstein, and who had died without a male heir. As the duchies, but not the Danish kingdom, were inherited after the salic law, the independence-movement had long nurished hopes that the king's death would lead to their goal.

Bismarck used the turbulence to invade the duchies in a Second war of Schleswig, and in the end both the independence-movement, Duke Frederick and all other interested powers were put aside, and the duchies incorporated into Prussia.