Saxe-Altenburg (German: Sachsen-Altenburg) was one of the Saxon duchies held by the Ernestine branch of the House of Wettin in present-day Thuringia.

The state had a constitutional monarchical form of government with a parliament composed of thirty members chosen by male taxpayers over 25 years of age.

The duchy consisted of two separate areas: the Ostkreis, containing the cities of Altenburg, Schmölln, Gößnitz, Lucka und Meuselwitz (including the exclave of Mumsdorf), Roschütz, Hilbersdorf, Neukirchen by Waldenburg and Rußdorf by Chemnitz; and the Westkreis, which contained the cities of Eisenberg, Kahla, Orlamünde und Roda (including the exclave of Ammelstädt).

The duchy had its origins in the medieval Burgraviate of Altenburg in the Imperial Pleissnerland (Terra Plisensis), a possession of the Wettin Margraves of Meissen since 1243.

It was an Imperial State in its own right, with a vote in the Reichstag, for much of the 17th century until the extinction of its ruling line in 1672 when it was inherited by Ernest I the Pious, the Duke of Saxe-Gotha, who had married the heiress.