Johann Ernst von Sachsen-Weimar or Johann Ernst of Saxe-Weimar can refer to:

• Johann Ernst I, Duke of Saxe-Weimar (1594-1626)
• Johann Ernst II, Duke of Saxe-Weimar (1627-1683)
• Johann Ernst III, Duke of Saxe-Weimar (1664-1707)
• Prince Johann Ernst of Saxe-Weimar (1696-1715), sometimes referred to as Johann Ernst IV of Saxe-Weimar; composer and younger son of Johann Ernst III

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