Captain Karl Mayr (January 5, 1883 in Mindelheim – February 9, 1945 in Buchenwald concentration camp) was a General Staff officer and Adolf Hitler's immediate superior in an army Intelligence Division in the Reichswehr, 1919-1920. Mayr was particularly known as the man who introduced Hitler to politics. In 1919, Mayr directed Hitler to write the Gemlich letter, in which Hitler first expressed his anti-semitic views in writing.

Mayr later became Hitler's opponent, and wrote in his memoirs that General Erich Ludendorff had personally ordered him to have Hitler join the Nazi Party and build it up. As far as it is known, his last rank was major. In 1933, he fled to France after the Nazis rose to power. Mayr was tracked down by the Gestapo, arrested, imprisoned, and later murdered at the Buchenwald Concentration Camp in 1945.

A fact-based portrayal of Mayr is dramatized in the 2002 film Max, a fictional account of Hitler's life in Munich just prior to joining the Nazi Party.

Mayr was the son of a magistrate. After graduating from high school, he was enrolled on 14 July 1901 in the 1st Bavarian Infantry Regiment in Munich as a cadet. Well regarded by his superiors, he made rapid progress, becoming Leutnant in 1903 and Oberleutnant in 1911. From August 1914 Mayr was with the 1st Bayerischen Jägerbattailon. During the First World War he was in combat in Lorraine and Flanders and involved in early 1915 with the German Alpine Corps. On 1 June 1915 Mayr was promoted to Hauptmann (captain). In 1917, he was named on the General Staff of the Alpine Corps. On 13 March 1918 he was appointed commander of the 1st Bavarian Jägerbattailon, with whom he served in the Eastern Army Group in Turkey from 20 July to 15 October 1918.