William Ernest was the last grand duke of Saxe-Weimar-Eisenach.

Saxe-Weimar-Eisenach was a German state, created as a duchy in 1809 by the merger of the Ernestine duchies of Saxe-Weimar and Saxe-Eisenach, which had been in personal union since 1741. It was raised to a grand duchy in 1815 by resolution of the Congress of Vienna. In 1903, it officially changed its name to the Grand Duchy of Saxony, but this name was rarely used. The grand duchy came to an end in the German Revolution of 1918–19 with the other monarchies of the German Empire. It was succeeded by the Free State of Saxe-Weimar-Eisenach, which was merged into the new Free State of Thuringia two years later.