Ernst I, Duke of Saxe-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.

Saxe-Altenburg was one of the Saxon duchies held by the Ernestine branch of the House of Wettin in present-day Thuringia. It was one of the smallest of the German states with an area of 1323 square kilometers and a population of 207,000 (1905) of whom about one fifth resided in the capital, Altenburg. The territory of the duchy consisted of two non-contiguous territories separated by land belonging to the Principality of Reuss-Gera. Its economy was based on agriculture, forestry, and small industry. The state had a constitutional monarchical form of government with a parliament composed of thirty members chosen by male taxpayers over 25 years of age.