Prince Wilhelm Karl Bernhard Hermann of Saxe-Weimar-Eisenach (21 December 1853 – 15 December 1924) was a member of the House of Saxe-Weimar-Eisenach.

Prince Wilhelm of Saxe-Weimar-Eisenach was born on 21 December 1853 in Stuttgart. He was the eldest son of the Prince Hermann of Saxe-Weimar-Eisenach and Princess Augusta of Württemberg (1826-1898). Prince Wilhelm also has had his own financial problems, and has been forced by the Grand Duke to live outside Weimar. Wilhelm is heir presumptive to the throne as the young Grand Duke Wilhelm Ernst is a widower. His wife, Karoline of Reuss died in January 1905. Many assume that the Grand Duke will marry again, and ensure the succession is kept to the main branch.

Prince William had a problem with his eldest son. Prince Hermann morganatically married Wanda Paola Lottero, an Italian stage actress, on 5 September 1909 in London. He rollicking lifestyle. The ducal family forced him to renounce his rights of succession to the Saxe-Weimar-Eisenach throne, as well as his royal status, title and prerogatives, granting him a lesser, noble title, Count Ostheim, along with a small allowance on the grounds that he stay out of the duchy. Prince Wilhelm also had a bad reputation. His behavior aroused the dissatisfaction of the head of the family. Prince Wilhelm fled to the United States in his youth, served as a riding master, clerk, book agent and even as a restaurant waiter in New York City, but was finally persuaded to return to Germany, marry his second cousin, and live on a small pension from the head of the house.

Prince Wilhelm married Gerta Princess of Ysenburg and Büdingen (1863-1945), daughter of Ferdinand Maximilian I. Prince of Ysenburg and Büdingen (1824-1903) and Auguste Marie Gertrude Princess of Hanau and Horowitz (1829-1887), on 11 April 1885 at Wächtersbach, Germany. Wilhelm and Gerta had three children:

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