John VIII, Count of Nassau-Siegen (Jan or Johan; Dillenburg, 29 September 1583 – Ronse, 27 July 1638) was a German nobleman and militarist of the 17th century.

On 25 December 1613, much to the horror of his family, he openly converted to Catholicism and entered in the service of the army of Charles Emmanuel I, Duke of Savoy.

John VIII of Nassau-Siegen is depicted in the famous painting The Surrender of Breda by Diego Velázquez, third from the left in the Spanish camp, looking directly at the spectator.

In 1631, he was in charge of the Spanish-Flemish Fleet defeated by the Dutch and Scots and English mercenaries, at the Battle of the Slaak, fighting on behalf of the Spanish Governor Francisco de Moncada, 3rd Marquis of Aitona, (1586–1635).

John VIII died in 1638 and was succeeded by his only son Johan Frans Desideratus, who had to cede part of Nassau-Siegen to the Protestant branch of the family.