Saxe-Gotha-Altenburg (German: Sachsen-Gotha-Altenburg) was a duchy ruled by the Ernestine branch of the House of Wettin in today's Thuringia, Germany.

The Duchy of Saxe-Gotha-Altenburg was nominally created in 1672, when Duke Frederick William III of Saxe-Altenburg died at the age of 14 and Ernest the Pious, by his marriage with Elisabeth Sophie, inherited the major part of his possessions.

He had the palaces and gardens in Gotha rebuilt in a lavish Baroque style and supported the religious refugees of the Moravian Church in Neudietendorf.

His sister Augusta married Prince Frederick of Wales in 1736, their first-born son George III was crowned King of Great Britain and Ireland in 1760.

Nevertheless, when the last dukes Emil August, a fervent admirer of the rise of Napoleon, and his brother Frederick IV had both died without male heirs, the house of Saxe-Gotha-Altenburg finally became extinct in 1825 and quarrels arose between the three remaining Ernestine lines about the succession.