Theodor von Sickel (18 December 1826 - 21 April 1908) was a German-Austrian historian who was born in Aken, Saxony-Anhalt. He specialized in early European medieval history, and is considered to be the founder of modern diplomatics (critical document research).
In 1850 he obtained his doctorate from the University of Halle, and furthered his studies at the École Nationale des Chartes in Paris. In 1857 he was appointed professor at the University of Vienna, and from 1869 to 1891 was director of the Institut für Österreichische Geschichtsforschung (Institute of Austrian Historical Research) which specialized in ancillary branches of historical study such as paleography, chronology and diplomatics. In the latter field, Sickel is acclaimed for his meticulous and expert edition of medieval documents, including over 1300 royal documents of the 10th century (Diplomata regum et imperatorum Germaniae)
In 1875 he became a member of the Monumenta Germaniae Historica, and from 1897 to 1908 was president of the Bavarian Academy of Sciences. In 1881 he established the Österreichisches Historisches Institut (Austrian Historical Institute) in Rome, and was its director until 1901.