Ernest Günther I of Schleswig-Holstein-Sonderburg-Augustenburg (14 October 1609 – 18 January 1689) was a Duke of Schleswig-Holstein of its Sonderburg line. He was the first to have his ducal seat in Augustenborg Palace, which he built and named in honor of his wife. He ruled from 1647 until his death in 1689.
Ernest Günther was born on 14 October 1609 at Haus Beck near Minden in Westphalia as the third son of Alexander, Duke of Schleswig-Holstein-Sonderburg and his wife, Countess Dorothea of Schwarzburg-Sondershausen (1579-1639). His father was the eldest son of John the Younger of Schleswig-Holstein-Sonderburg, who himself was a younger son of King Christian III of Denmark-Norway. Ernest Günther thus belonged to a junior branch of the House of Oldenburg which had ruled Denmark since 1448.
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). They had ten children: