John VIII of Nassau-Siegen (Jan or Johan), Count of Nassau-Siegen, Katzenelnbogen, Vianden and Dietz, marquis of Monte-Caballo, baron of Ronse and baron of Beilstein (Dillenburg, September 29, 1583 – Ronse, July 27, 1638) was a Dutch noble and military of the 17th century.
John VIII was the second son of John VII, Count of Nassau and Countess Magdalena of Waldeck. He was educated in Herborn, Kassel and Geneva. In 1610 he participated in the Dutch States Army in the conquest of Jülich.
On December 25, 1613, much to the horror of his family, he openly converted to Catholism and entered in the service of the army of Charles Emmanuel I, Duke of Savoy. After the death of his elder brother, Johann Ernst in september 1617, he claimed his rights, but his father chose a Protestant successor. When his father died in 1623, John VIII occupied Nassau-Siegen at the head of a Habsburg Army and started the Contra-Reformation.
In 1624 he became a Knight in the Order of the Golden Fleece.
In 1625 he participated in the Siege of Breda (1624), defended for 11 months before surrendering, by his second cousin Justinus van Nassau.
John VIII of Nassau-Siegen is depicted on the famous portrait The Surrender of Breda by Diego Velázquez, third from the left in the Spanish camp, looking straight at the spectator.
In 1630 he was captured by the Dutch Army but released the same year. 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).
In 1632, Nassau-Siegen was conquered by the Swedes, after which his half-brother John Maurice of Nassau-Siegen re-introduced Protestantism.
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.
John married on augustus 13 1618 in Brussels with the Catholic Ernestine Yolande de Ligne D'amblise (1594–1668), daughter of Lamoral de Ligne, Prince d'Espinoy. They had six children.