Mathilde Friederike Karoline Ludendorff (born Mathilde Spiess; 4 October 1877 in Wiesbaden – 24 June 1966 in Tutzing) was a German psychiatrist. Her third husband was General Erich Ludendorff. She was a leading figure in the Völkisch movement known for her esoteric and conspiratorial ideas. Together with Ludendorff, she founded the (German) ( translated: Society for the Knowledge of God), a small and rather obscure esoterical society of theists, which was banned from 1961 to 1977.
Mathilde Spiess was born in Wiesbaden, Hesse in central Germany, the daughter of Bernhard Spiess, a Lutheran minister. She attended a private and a public school for girls. Despite their modest means, the parents enabled their daughters to a practical professional education which was unusual at the time. From 1893 until 1895 she trained to be a school teacher for girls. From 1896 onward she taught initially at a boarding school for girls in Biebrich (Wiesbaden). After she had saved enough money, she attended evening school at the from 1900 until 1901 for her Abitur.
During the winter semester of 1901/1902 she began to study medicine at the Albert Ludwigs Universität in Freiburg, where she heard August Weismann lecture (Vorlesungen über Deszendenztheorie) amongst others.
In 1904, she married her lecturer, anatomist Gustav Adolf von Kemnitz, and moved to Munich in 1905, where she had a daughter, Ingeborg von Kemnitz (1906–1970) and the twins Asko (1909–1992) and Hanno (1909–1990). Two years later in 1911, she picked up her interrupted medical studies in Munich until 1912, followed by the Medizinalpraktikum (practical year) part time in the gynecology department of the Universitätsklinik Bonn, followed by approbation in 1913. She also graduated in 1913 with a PhD degree in neurology with a thesis examining the hereditary nature of mental differences between genders.