Ernest I or Ernst I can refer to:
- Ernest I, Prince of Anhalt-Dessau (d. 1516)
- Ernest I, Duke of Brunswick-Lüneburg (1497–1546), Ernest the Confessor
- Ernst I, Duke of Saxe-Coburg-Altenburg (1601–1675), Ernest the Pious
- Ernst I, Duke of Saxe-Coburg and Gotha (1784–1844), father of Price Albert, the consort of Queen Victoria
- Ernest I, Duke Brunswick-Grubenhagen, 14th century