Prince Hermann of Saxe-Weimar-Eisenach (14 February 1886 – 6 June 1964) was a member of the House of Saxe-Weimar-Eisenach. He was heir to his relative William Ernest, Grand Duke of Saxe-Weimar-Eisenach until 1909, when he was disinherited of his royal status. From that point onwards, Hermann was commonly referred to with the lesser style, Count Ostheim.

Prince Hermann of Saxe-Weimar-Eisenach was born on 14 February 1886 in Düsseldorf. He was educated by a tutor until deemed old enough to enter the German army. He joined the Cuirassiers of the Guard in Berlin, where he was separated from the guidance of his family and tutor, and began to build up a reputation as a spendthrift like his father. He was given $10,000 a year to spend, and he and those he bought items from realized that any debts contracted would eventually be paid by his family, thus increasing the amount Hermann could spend. By the end of the year, Hermann was a quarter of a million dollars in debt, which his family duly paid; he was sent to a small town as a disciplinary measure. He persuaded his family that he was ill, and was able to travel to Paris, racking up more debts along the way; one rumor said he sold his mother's jewels en route to France.

William Ernest, Grand Duke of Saxe-Weimar-Eisenach remained childless for much of his early life, fueling speculation of the succession to his duchy. As a descendant of Charles Augustus, Grand Duke of Saxe-Weimar-Eisenach through a younger son, firstly Hermann and secondly his brother were heir presumptives until the birth of Charles Augustus, Hereditary Grand Duke of Saxe-Weimar-Eisenach in 1912.

A lifelong spendthrift, Prince Hermann was heir presumptive to the duchy of Saxe-Weimar-Eisenach until his disinheritance on 2 August 1909. 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. Herman was not the only member of his family to have a bad reputation; his father Prince William as well as their cousin Prince Bernhard were all viewed with displeasure, so much so, that the still-living Prince William had been overlooked concerning the duchy's succession. Hermann had a younger brother, Prince Albert, who took up his position as next-in-line to the duchy. Hermann was also driven out of the German army "for all sorts of unsavory scraps", as he was wanted in both England and Austria for debts, and for being a "common swindler". His Austrian arrest warrant was issued soon after his younger sister Princess Sophie of Saxe-Weimar-Eisenach was denied permission to enter into a morganatic marriage; she committed suicide soon after, on 18 September 1913.

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