| Ernest III, Duke of Brunswick-Grubenhagen | |
| House: | House of Guelph |
| Father: | Philip I, Duke of Brunswick-Grubenhagen |
| Mother: | Countess Catherine of Mansfeld-Vorderort |
| Spouse: | Princess Margaret of Pomerania-Wolgast |
| Issue: | Elisabeth |
| Birth Place: | Osterode am Harz |
| Death Place: | Herzberg am Harz |
Ernest III of Brunswick-Grubenhagen-Herzberg (by a different counting: Ernest IV; 17 December 1518 in Osterode am Harz - 2 April 1567 in Herzberg Castle, Herzberg am Harz), was a member of the noble family of Guelph and a duke of Brunswick-Grubenhagen.
Ernest was the eldest son of Duke Philip I of Brunswick-Grubenhagen (1476–1551) and his second wife, Countess Catherine of Mansfeld-Vorderort (1501–1535), eldest daughter of Count Ernest II and his first wife Barbara of Querfurt. His paternal grandparents were Duke Albert II of Brunswick-Grubenhagen and his wife Countess Elisabeth of Waldeck.
Together with his father and brothers, he took part in a military campaign in southern Germany in 1546, during the Schmalkaldic War, which ended unsuccessfully at Ingolstadt.
On 9 October 1547 in Wolgast, Duke Ernest III married Princess Margaret of Pomerania-Wolgast (1518–1569), eldest daughter of Duke George I of Pomerania and his first wife, Princess Amalia of the Palatinate. From this marriage only one daughter reached adulthood:
married in August 1568 Duke John II of Schleswig-Holstein-Sonderburg (1545–1622). That marriage produced 14 children:
* Dorothea (1569–1593) married Duke Frederick IV of Legnica, Silesia
* Christian (1570–1633), Duke of Schleswig-Holstein-Sonderburg-Ærø
* Ernest (1572–1596), Duke of Schleswig-Holstein-Sonderburg
* Alexander (1573–1627), Duke of Schleswig-Holstein-Sonderburg, married Countess Dorothea of Schwarzburg-Sondershausen
* August (1574–1596), Duke of Schleswig-Holstein-Sonderburg
* Mary (1575–1640), abbess of Itzehoe
* John Adolph (1576–1624), Duke of Schleswig-Holstein-Norburg
* Anna (1577–1610) married 1601 Duke Bogislaw XIII of Pomerania
* Sophie (1579–1618) married 1607 Duke Philip II of Pomerania-Stettin
* Elizabeth (1580–1653) married 1615 Duke Bogislaw XIV of Pomerania
* Frederick (1581–1658), Duke of Schleswig-Holstein-Norburg married firstly in 1627 princess Juliana of Saxe-Lauenburg and secondly in 1632 Eleanore Princess of Anhalt-Zerbst
* Margaret (1583–1638) married 1603 Count John VII of Nassau-Siegen
* Philip (1584–1663), Duke of Schleswig-Holstein-Glücksburg married Princess Sophie Hedwig of Saxe-Lauenburg
* Albert (1585–1613), Duke of Schleswig-Holstein-Sonderburg
Because Ernest had no sons, after his death, his duchy was inherited by his younger brother Wolfgang. When he, too, died without a male heir, the duchy was inherited by their youngest brother, Philip II. With Philip's death, the Brunswick-Grubenhagen line of the House of Guelph died out, and the duchy was merged back into Brunswick-Wolfenbüttel.