Frederick I, Duke of Saxe-Gotha-Altenburg

Frederick I, Duke of Saxe-Gotha-Altenburg (15 July 1646 Gotha, Duchy of Saxe-Gotha – 2 August 1691 Friedrichswerth), was a duke of Saxe-Gotha-Altenburg. He was the fourth but eldest surviving son of Ernst I, Duke of Saxe-Coburg-Altenburg and Elisabeth Sophie of Saxe-Altenburg.

Frederick I
Duke of Saxe-Gotha-Altenburg
Reign	1675–1691
Predecessor	New creation
Successor	Frederick II
Born	(1646-07-15)15 July 1646 Gotha, Duchy of Saxe-Gotha
Died	2 August 1691(1691-08-02) (aged 45) Friedrichswerth
Spouse	Magdalena Sibylle of Saxe-Weissenfels Christine of Baden-Durlach
Issue among others...	Anna Sophie, Princess of Schwarzburg-Rudolstadt Dorothea Marie, Duchess of Saxe-Meiningen Frederick II
House	House of Saxe-Gotha-Altenburg
Father	Ernst I, Duke of Saxe-Coburg-Altenburg
Mother	Elisabeth Sophie of Saxe-Altenburg
Religion	Lutheran

When Ernst inherited the duchy of Saxe-Altenburg (1672), he made Frederick the regent of that duchy. In 1674 Ernst, who was already ill, made Frederick the regent of his entire lands.

After the death of his father (1675) Frederick assumed the throne of both duchies. However, on the basis of his family's house law, he had to allow his six younger brothers to take part in the government. At first, they agreed to a common household of all seven brothers in the Schloss Friedenstein, though this arrangement endured only until 1676.

Afterwards, negotiations began for the division of the paternal inheritance. This was finally accomplished on 24 February 1680; Frederick kept Gotha, Tenneberg, Wachsenburg, Ichtershausen, Georgenthal, Schwarzwald, Reinhardsbrunn, Volkenrode, Oberkranichfeld, Orlamünde, Altenburg and Tonna. These towns virtually formed the old duchy of Saxe-Gotha-Altenburg. They consisted of three large and coherent areas around Gotha, Kahla and Altenburg, as well as six smaller enclaves.

Frederick continued the work of his father. In order to prevent future disputes between his descendants, he established primogeniture for his house in 1685 (with Imperial assent granted in 1688). Around 1680 he established himself in the Lustschloss Friedrichswerth, near the village of Erffa, approximately 20 km of Gotha, which was renamed in his honour Friedrichswerth.

In 1683 Frederick created the (still in existence today) Theatre of Gotha (Gothaer Schloßtheater). He was also an eager diary writer; these diaries became one of the most important sources of his time. Frederick took part in the Great Turkish War against the Turks, and in the War of the Grand Alliance against France. He ruined the finances of his small duchy, however, using them to maintain a standing army, which by the time of his death counted over 10,000 men.