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.

He was of a retiring disposition and he took little active part in running the country.

After a reign that lasted 55 years, he died without a living direct male heir; because of this, he was succeeded by his nephew, Ernst.

Agnes was a sister of Friedrich I, Duke of Anhalt.