Saxe-Altenburg (Sachsen-Altenburg) was one of the Saxon duchies held by the Ernestine branch of the Wettin dynasty. Altenburg was its own state, with a vote in the diet, for much of the 17th century until the extinction of its ruling line in 1672, when it was inherited by the Duke of Saxe-Gotha, who married the heiress. It remained part of Saxe-Gotha-Altenburg until the extinction of that house in 1825 , when Gotha and Altenburg were split up, with Gotha going to the Duke of Saxe-Coburg-Saalfeld and Altenburg to the Duke of Saxe-Hildburghausen, who in exchange gave up Hildburghausen to the Duke of Saxe-Meiningen. This family ruled in the duchy until the end of the monarchies in 1918. Saxe-Altenburg was incorporated into the new state of Thuringia in 1920.
Saxe-Altenburg had an area of 1,323 km² and a population of 207,000 (1905). Its capital was Altenburg.
The Saxe-Altenburg line became extinct following the death of Prince George Moritz in 1991.
In 1991 the Saxe-Altenburg line became extinct.
Two branches descend from duke Ernest the Pious, the father of the progenitor of this Saxe-Altenburg branch: Saxe-Meiningen and Saxe-Coburg and Gotha; according to old Wettin family law, they would have divided the actual territories between them (as happened to Gotha and Altenburg in 1826).