Karl August, Grand Duke of Saxe-Weimar-Eisenach (3 September 1757 – 14 June 1828) was a Duke of Saxe-Weimar and of Saxe-Eisenach (in personal union) from 1758, Duke of Saxe-Weimar-Eisenach from its creation (as a political union) in 1809, and grand duke from 1815 until his death. He is noted for the intellectual brilliance of his court.