Saxe-Gotha-Altenburg (Sachsen-Gotha-Altenburg) was a historical state in today's Thuringia, Germany. It was nominally created in 1672 when Frederick Wilhelm III, the last duke of Saxe-Altenburg, died and Ernest I, Duke of Saxe-Gotha (who had married Frederick Wilhelm's cousin, Elisabeth Sophie), inherited his possessions.

It was common for the Ernestine duchies to merge and split; Ernest's combined duchy was divided again after his death in 1675, and the Duchy of Saxe-Gotha-Altenburg officially came into existence with the completion of this division and the accession in 1680 of his eldest son, Frederick, to the subdivision centered around Gotha and Altenburg.

When the house of Saxe-Gotha and Altenburg became extinct in 1825, Saxe-Gotha was given to Saxe-Coburg-Saalfeld and Saxe-Altenburg passed to the Duke of Saxe-Hildburghausen, who in turn passed his own domain to Saxe-Meiningen. After the abolition of German monarchies at the end of the First World War, both Saxe-Gotha and Saxe-Altenburg became part of the newly created state of Thuringia in 1920.

Dukes of Saxe-Gotha-Altenburg

• Ernest I the Pious (1640-1675), inherited Saxe-Altenburg in 1675
• Frederick I (1675-1691), son of previous; first to bear the title Duke of Saxe-Gotha-Altenburg
• Frederick II (1691-1732), son
• Frederick III (1732-1772), son
• Ernest II (1745-1804), son
• Emil (1804-1822), son
• Frederick IV (1822-1825), brother

Divided between Saxe-Coburg-Saalfeld and Saxe-Hildburghausen