Saxe-Altenburg (German 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 1675, when it was inherited by the Duke of Saxe-Gotha, who married the heiress. It remained part of Saxe-Gotha 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 line 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.