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.
^ abStaatshandbücher für das Herzogtum Sachsen-Altenburg (1902), "Genealogie des Herzoglichen Hauses Sachsen-Altenburg" pp. 1-2
^Staatshandbuch für das Großherzogtum Sachsen / Sachsen-Weimar-Eisenach (1851), "Großherzogliche Hausorden" p. 8Archived 2020-07-06 at the Wayback Machine