Saxe-Gotha-Altenburg was a duchy ruled by the Ernestine branch of the House of Wettin in today's Thuringia, Germany. The extinction of the line in 1825 led to a major re-organisation of the Thuringian states.

The Ernestine duchies, also known as the Saxon duchies, were a group of small states whose number varied, which were largely located in the present-day German state of Thuringia and governed by dukes of the Ernestine line of the House of Wettin.