|Intro||Duke of Saxe-Eisenach|
|A.K.A.||Johann Wilhelm von Sachsen-Eisenach|
|Birth||17 October 1666, Friedewald, Altenkirchen district, Rhineland-Palatinate, Germany|
|Death||14 January 1729, Eisenach, Thuringia, Germany (aged 62 years)|
John William III, Duke of Saxe-Eisenach (Friedewald, 17 October 1666 – Eisenach, 14 January 1729), was a duke of Saxe-Eisenach.
He was the third son of John George I, Duke of Saxe-Eisenach and Johannetta of Sayn-Wittgenstein. His twin brother, Maximilian, died at the age of two years.
He succeeded his brother John George II as duke of Saxe-Eisenach when his brother died in 1698.
In Oranjewoud on 28 November 1690, John William married with Amalie (The Hague, 25 November 1655 – Allstedt, 16 February 1695), a daughter of William Frederick, Prince of Nassau-Dietz. They had two children:
- Wilhelm Heinrich, Duke of Saxe-Eisenach (b. Oranjewoud, 10 November 1691 - d. Eisenach, 26 July 1741).
- Albertine Johannetta (b. Oranjewoud, 28 February 1693 - d. Eisenach, 1 April 1700).
In Wolfenbüttel on 27 February 1697 — two years of the death of his first wife — John William married secondly with Christine Juliane of Baden-Durlach, a daughter of Charles Gustav of Baden-Durlach. They had seven children:
- Johannette Antoinette Juliane (b. Jena, 31 January 1698 - d. Schloss Dahme, 13 April 1726), married on 9 May 1721 to Duke Johann Adolf II of Saxe-Weissenfels.
- Karoline Christine (b. Jena, 15 April 1699 - d. Philippsthal, 25 July 1743), married on 24 November 1725 to Landgrave Charles I of Hesse-Philippsthal.
- Anton Gustav (b. Eisenach, 12 August 1700 - d. Eisenach, 4 October 1710).
- Charlotte Wilhelmine Juliane (b. Eisenach, 27 June 1703 - d. Erfurt, 17 August 1774).
- Johannetta Wilhelmine Juliane (b. Eisenach, 10 September 1704 - d. Eisenach, 3 January 1705).
- Karl Wilhelm (b. Eisenach, 9 January 1706 - d. Eisenach, 24 February 1706).
- Karl August (b. Eisenach, 10 June 1707 - d. Eisenach, 22 February 1711).
In Weissenfels on 28 July 1708 — one year after the death of his second wife — John William married thirdly with Magdalene Sibylle of Saxe-Weissenfels, a daughter of Johann Adolf I, Duke of Saxe-Weissenfels. They had three children:
- Johanna Magdalene Sophie (b. Eisenach, 19 August 1710 - d. Eisenach, 26 February 1711).
- Christiane Wilhelmine (b. Altenkirchen, 3 September 1711 - d. Idstein, 27 November 1740), married on 26 November 1734 to Prince Charles of Nassau-Usingen.
- Johann Wilhelm (b. Marksuhl, 28 January 1713 - d. Eisenach, 8 May 1713).
In the Schloss Philippsruhe on 29 May 1727 — one year after the death of his third wife — John William married fourthly with Marie Christine Felizitas of Leiningen-Dagsburg-Falkenburg-Heidesheim, Dowager Princess of Baden-Durlach. This union was childless.