Otto Weininger (German: [ˈvaɪnɪŋɐ]; 3 April 1880 – 4 October 1903) was an Austrian philosopher who in 1903 published the book Geschlecht und Charakter (Sex and Character), which gained popularity after his suicide at the age of 23. Weininger had a strong influence on Ludwig Wittgenstein, August Strindberg, and, via his lesser-known work Über die letzten Dinge, on James Joyce.

Otto Weininger was born on 3 April 1880 in Vienna, a son of the Jewish goldsmith Leopold Weininger and his wife Adelheid. After attending primary school and graduating from secondary school in July 1898, Weininger registered at the University of Vienna in October of the same year. He studied philosophy and psychology but took courses in natural sciences and medicine as well. Weininger learned Greek, Latin, French and English very early, later also Spanish and Italian, and acquired passive knowledge of Swedish, Danish and Norwegian. While he was at the university, he would attend the Philosophical Society, where he heard, among others, Richard Wagner's son-in-law Houston Stewart Chamberlain, who was regarded as an outsider but an original thinker.

In the autumn of 1901, Weininger tried to find a publisher for his work Eros and the Psyche: A biological-psychological study, which he submitted to his professors Friedrich Jodl and Laurenz Müllner [de] as his thesis in 1902. He met Sigmund Freud, who did not, however, recommend the text to a publisher. His professors accepted the thesis and Weininger received his Ph.D. degree in July 1902. Shortly thereafter he became a Protestant.

In 1902 Weininger went to Bayreuth, where he witnessed a performance of Richard Wagner's Parsifal, which impressed him deeply. Via Dresden and Copenhagen he made his way to Christiania (Oslo), where he saw for the first time Henrik Ibsen's Peer Gynt on stage. Upon his return to Vienna, Weininger suffered from fits of deep depression. The decision to take his own life gradually took shape; after a long discussion with his friend Artur Gerber, however, Weininger realized that "it is not yet time".

In June 1903, after months of concentrated work, his book Sex and Character: A Fundamental Investigation – an attempt "to place sex relations in a new and decisive light" – was published by the Vienna publishers Braumüller & Co. The book contained his thesis to which three vital chapters were added: (XII) "The Nature of Woman and her Relation to the Universe", (XIII) "Judaism", (XIV) "Women and Humanity".

Although the book's reception was not negative, it did not create the expected stir. Weininger was attacked by Paul Julius Möbius, professor in Leipzig and author of the book On the Physiological Deficiency of Women and was accused of plagiarizing. Deeply disappointed and seemingly depressed, Weininger left for Italy.

Back in Vienna, he spent his last five days with his parents. On 3 October, he took a room in the house at Schwarzspanierstraße 15, where Ludwig van Beethoven had died. He told the landlady that he was not to be disturbed before morning, since he planned to work and then to go to bed late. That night he wrote two letters, one to his father and the other to his brother Richard, telling them that he was going to shoot himself.

On 4 October, Weininger was found mortally wounded, having shot himself in the chest. He died in the Wiener Allgemeines Krankenhaus (Vienna General Hospital) and was buried in the Matzleinsdorf Protestant Cemetery in Vienna.