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Saxe-Gotha-Altenburg was a duchy in today's Thuringia, Germany. It was ruled by the Ernestine branch of the House of Wettin. The duchy existed from 1672 to 1826.
It was formed, when Frederick Wilhelm III, the last duke of Saxe-Altenburg died and Ernest I, Duke of Saxe-Gotha inherited his possessions.
In 1825 the royal house of Saxe-Gotha and Altenburg extinct. In 1825 the royal house of Saxe-Gotha and Altenburg extinct. That was the reason, that Saxe-Gotha was given to Saxe-Coburg-Saalfeld and Saxe-Altenburg passed to the Duke of Saxe-Hildburghausen.
In 1920, all former duchies became part of the newly created state of Thuringia.