William Ernest, Duke of Saxe-Weimar

Wilhelm Ernst, Duke of Saxe-Weimar (19 October 1662 – 26 August 1728) was a duke of Saxe-Weimar.

He was born in Weimar, the eldest son of Johann Ernst II, Duke of Saxe-Weimar and Christine Elisabeth of Holstein-Sonderburg.

When his father died in 1683, he succeeded him as duke; however, he was compelled to rule jointly with his younger brother Johann Ernst III.

Because Johann Ernst III was alcoholic, Wilhelm Ernst took full control of the government of the duchy and permitted Johann Ernst the nominal title of co-duke (Mitherr) until his death in 1707. After the death of his brother he made Johann Ernst's son, Ernst August I, co-duke, but with no real power.

Six months after the death of his father (2 November 1683), Wilhelm Ernst married in Eisenach with Charlotte Marie, his cousin and eldest surviving daughter of his uncle Bernhard II, Duke of Saxe-Jena, in order to secure the family lands. At that time, the guardian of Charlotte and his younger brother, the duke Johann Wilhelm of Saxe-Jena, was the duke Johann Georg I of Saxe-Eisenach, their only surviving uncle. When he died in 1686, the guardianship of the duke of Saxe-Jena was taken by Wilhelm Ernst, his cousin and brother-in-law.

Wilhelm Ernst was a strict Lutheran and commanded that only men who could read and comment on Lutheran theological writings be admitted to his armed forces. The composer Johann Sebastian Bach worked for the duke from 1708, first as organist, then as Konzertmeister (leader of the orchestra) in Weimar. When Johann Samuel Drese died in 1716, Bach solicited his post of Kapellmeister (head of the court musical establishment), but Wilhelm Ernst appointed Drese's incompetent son for the post instead; furious, Bach solicited his dismissal from the Duke's service. Annoyed at Bach's impertinence, Wilhelm Ernst had Bach jailed for four weeks in a fortress before he accepted his dismissal.

On 23 August 1690, Wilhelm Ernst and Charlotte Marie were divorced after seven years of childless and extremely unhappy union. Two months later, on 4 November, duke Johann Wilhelm of Saxe-Jena (Charlotte Marie's brother), died without heirs, and Wilhelm Ernst took possession of part of his duchy after a treaty was signed with the Saxe-Eisenach branch of his family.

Wilhelm Ernst never remarried and died at Weimar without heirs; his nephew Ernst August I became his successor.