– 23 November 1639), German: Johann Ernst Graf von Nassau-Siegen, official titles: Graf zu Nassau, Katzenelnbogen, Vianden und Diez, Herr zu Beilstein, was a count from the House of Nassau-Siegen, a cadet branch of the Ottonian Line of the House of Nassau.

[2][note 2] as the seventh and youngest son of Count John VII 'the Middle' of Nassau-Siegen and his second wife, Duchess Margaret of Schleswig-Holstein-Sonderburg.

[8][9] After his older half-brother John 'the Younger' had accepted the homage of the city of Siegen for the entire county of Nassau-Siegen on 12 January 1624[10] and had voluntarily ceded the sovereignty over the Hilchenbach district with Ginsburg Castle [de] and some villages belonging to the Ferndorf [de] and Netphen districts to his younger brother William on 13/23 January 1624,[11][12] John Ernest and his brothers, with the exception of the oldest two brothers John Maurice and George Frederick, accepted only modest appanages.

[13][14] John Ernest enlisted in the Dutch States Army under Prince Frederick Henry of Orange.

[17][18] He died of dysentery[6][18] in São Salvador da Bahia de Todos os Santos[2][6] on 23 November 1639[19] aboard the 'Alkmaar', and was buried in the Calvinist church in Mauritsstad on 1 December.