Joachim Ernest, Duke of Schleswig-Holstein-Sonderburg-Plön

John was born in Haderslev as the fourth child (third son) of King Christian III of Denmark and his wife Dorothea of Saxe-Lauenburg. His elder brother king Frederick II of Denmark allotted him a portion of Schleswig, Sonderburg (today Danish Sønderborg) as his ducal seat. He acted actively in the Danish government, for example supporting his sister-in-law the queen when his nephew Christian IV of Denmark was underage. There were plans for him to marry the dowager queen in 1588 or 1589, repudiating his second, young wife. John died in Glücksburg.

John married twice. Firstly, he married in Kolding on August 19, 1568 Elisabeth of Brunswick-Grubenhagen (20 March 1550 Salzderhelden – 11 February 1586 Østerholm), daughter of Ernest III, Duke of Brunswick-Grubenhagen. They had the following children:

1. Dorothea (9 October 1569 Kolding– 5 July 1593 Legnica), married on 23 November 1589 to Duke Frederick IV of Legnica
2. Duke Christian of Schleswig-Holstein-Sønderborg-Ærø (1622–33) (24 October 1570 – 4 June 1633)
3. Ernest (17 January 1572 – 26 October 1596)
4. Duke Alexander of Schleswig-Holstein-Sønderborg (1622–27) (20 January 1573 – 13 May 1627)
5. August (26 July 1574 – 26 October 1596)
6. Marie, Abbess of Itzehoe (22 August 1575 – 6 December 1640)
7. Duke John Adolf of Schleswig-Holstein-Sønderborg-Nordborg (1622–24) (17 September 1576 – 21 February 1624)
8. Anna (7 October 1577 – 30 January 1616), married on 31 May 1601 to Duke Bogislaw XIII of Pomerania-Barth
9. Sophia (30 May 1579 – 3 June 1658, in Treptow an der Rega, her dower), married on 8 March 1607 to Duke Philip II of Pomerania-Barth
10. Elisabeth (24 September 1580 – 21 December 1653), married on 19 February 1615 to Duke Bogislaw XIV of Pomerania-Rügenwalde
11. Duke Frederick of Schleswig-Holstein-Sønderborg-Nordborg (1624–58) (26 October 1581 – 22 July 1658), married on 1 August 1627 Juliana of Saxony, Angria and Westphalia (Lauenburg), daughter of Duke Francis II
12. Margarete (24 February 1583 – 10 April 1638), married on 27 February 1603 to Count John VII of Nassau-Siegen
13. Duke Philipp of Schleswig-Holstein-Sønderborg-Glücksburg (1622–63) (15 March 1584 – 27 September 1663)
14. Albrecht (16 April 1585 – 30 April 1613)

Secondly, he married on 14 February 1588 Princess Agnes Hedwig of Anhalt (12 March 1573 Dessau – 3 November 1616 Sønderborg) and they had the following children:

1. Eleonore (4 April 1590 – 13 April 1669)
2. Anna Sabine (7 March 1593 – 18 July 1659), married on 1 January 1618 to Duke Julius Frederick of Württemberg-Weiltingen
3. Johann Georg (9 February 1594 – 25 January 1613)
4. Duke Joachim Ernst I of Schleswig-Holstein-Sønderborg-Plön (1622–71) (29 August 1595 – 5 October 1671)
5. Dorothea Sibylle (13 July 1597 – 21 August 1597)
6. Dorothea Marie (23 July 1599 – 27 March 1600)
7. Bernhard (12 April 1601 – 26 April 1601)
8. Agnes Magdalene (17 November 1602 – 17 May 1607)
9. Eleonore Sofie (24 February 1603 – 5 January 1675), married on 28 February 1624 to Christian II, Prince of Anhalt-Bernburg