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Johann Ernst III, Duke of Saxe-Weimar (b. Weimar, 22 June 1664 - d. Weimar, 10 May 1707), was a duke of Saxe-Weimar.

He was the second son of Johann Ernst II, Duke of Saxe-Weimar and Christine Elisabeth of Holstein-Sonderburg.

After the death of his father (1683), he inherited the duchy of Saxe-Weimar with his older brother Wilhelm Ernst as co-ruler (Mitherr).

Johann Ernst was an alcoholic; this, and his non-interest in the government, was taken advantage of by his brother, who became the only, autocratic, ruler of the duchy. However, until the time of his death, Johann Ernst served as co-duke, without any significant influence on the government.

In Zerbst on 11 October 1685 Johann Ernst married firstly Sophie Auguste of Anhalt-Zerbst. They had five children:

1. Johann Wilhelm (b. Weimar, 4 June 1686 - d. Weimar, 14 October 1686).
2. Ernst August I, Duke of Saxe-Weimar-Eisenach-Jena (b. Weimar, 19 April 1688 - d. Eisenach, 19 January 1748).
3. Eleonore Christiane (b. Weimar, 15 April 1689 - d. Weimar, 7 February 1690).
4. Johanna Auguste (b. Weimar, 6 July 1690 - d. Weimar, 24 August 1691).
5. Johanna Charlotte (b. Weimar, 23 November 1693 - d. Weimar, 2 March 1751).

In Kassel on 4 November 1694 and only two months after the death of his first wife, Johann Ernst married secondly Charlotte Dorothea Sofie of Hesse-Homburg. They had four children:

1. Karl Frederick (b. Weimar, 31 October 1695 - d. Weimar, 30 March 1696).
2. Johann Ernst (b. Weimar, 25 December 1696 - d. Frankfurt, 1 August 1715).
3. Marie Luise (b. Weimar, 18 December 1697 - d. Weimar, 29 December 1704).
4. Christiane Sophie (b. Weimar, 7 April 1700 - d. Weimar, 18 February 1701).

After Johann Ernst's death, his successor and eldest son, Ernst August, was nominally given his father's power, but the real power was retained by his uncle, Wilhelm Ernst, until his death in 1728, when Ernst August, became the sole reigning duke of Saxe-Weimar.