| Ernst I | |
| Full Name: | Ernst Frederick Paul Georg Nikolaus |
| Succession: | Duke of Saxe-Altenburg |
| Reign: | 3 August 1853 – 7 February 1908 |
| Predecessor: | George |
| Successor: | Ernst II |
| Issue: | Marie, Princess Albert of Prussia |
| House: | Wettin |
| Father: | Georg, Duke of Saxe-Altenburg |
| Mother: | Marie Luise of Mecklenburg-Schwerin |
| Birth Date: | 16 September 1826 |
| Birth Place: | Hildburghausen |
| Death Place: | Altenburg |
| Religion: | Lutheranism |
Ernst I, Duke of Saxe-Altenburg (German: Ernst I. Friedrich Paul Georg Nikolaus von Sachsen-Altenburg) (16 September 1826 in Hildburghausen – 7 February 1908 in Altenburg), was a duke of Saxe-Altenburg. He was the first son of Georg, Duke of Saxe-Altenburg and Marie Luise of Mecklenburg-Schwerin. In 1853 he succeeded his father as Duke of Saxe-Altenburg. He was of a retiring disposition and he took little active part in running the country. After a reign that lasted fifty five years, he died without a living direct male heir; because of this, he was succeeded by his nephew, Ernst.
He was married in Dessau on 28 April 1853 to Agnes of Anhalt-Dessau (1824–1897). Agnes was a sister of Friedrich I, Duke of Anhalt.[1]
They had two children:
Belgium: Grand Cordon of the Royal Order of Leopold
Denmark: Knight of the Elephant, 28 October 1882[4]
Netherlands: Grand Cross of the Netherlands Lion
Sweden-Norway: Knight of the Seraphim, 24 February 1864[5]