Ernest Günther I of Schleswig-Holstein-Sonderburg-Augustenburg (14 October 1609 – 18 January 1689) was a Duke of Schleswig-Holstein of its Sonderburg line.

[1] He was the first to have his ducal seat in Augustenborg Palace, which he built and named in honor of his wife.

[3] He was third son of Alexander, Duke of Schleswig-Holstein-Sonderburg and his wife, Countess Dorothea of Schwarzburg-Sondershausen (1579-1639).

[1] On 15 June 1651 he married Princess Auguste of Schleswig-Holstein-Sonderburg-Glücksburg (27 June 1633 – 26 May 1701), daughter of Duke Philipp of Schleswig-Holstein-Sonderburg-Glücksburg and his wife, Princess Sophie Hedwig of Saxe-Lauenburg (1601-1660).

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