Ernest I, called Ernest the Pious (German: Ernst I., der Fromme; 25 December 1601 – 26 March 1675), was duke of Saxe-Gotha and Saxe-Altenburg, later united as Saxe-Gotha-Altenburg. He was a surviving son of Johann II, Duke of Saxe-Weimar and Dorothea Maria of Anhalt. He is remembered for rebuilding and reforming his lands after the Thirty Years' War. A devout Lutheran, he allied with Sweden in 1631 and fought at Lech, Nördlingen, Lützen, and the siege of Nuremberg; after the Peace of Prague (1635) he withdrew from warfare to focus on administration and recovery.
With Veit Ludwig von Seckendorf and Andreas Reyher, he led major educational reforms through the Schulmethodus (1642), promoting compulsory and graded schooling with a broader curriculum. He also founded the ducal library at Gotha and patronized early currents of the German Enlightenment.