Saxe-Gotha-Altenburg (German: Sachsen-Gotha-Altenburg) was a country in what is today Thuringia, Germany.
It was formed 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.
The duchy was split up again after Ernest’s death in 1675, and the Duchy of Saxe-Gotha-Altenburg officially came into existence in 1680 when his eldest son, Frederick, took control of the parts of the country Gotha and Altenburg.