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. 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.