Karl Harrer (8 October 1890 - 5 September 1926) was a German journalist and politician, one of the founding members of the "Deutsche Arbeiterpartei" ("German Workers' Party", DAP) in 1919, the party that soon would become the Nationalsozialistische Deutsche Arbeiterpartei (NSDAP).
Harrer was also a member of the Thule Society, which gave him the task of founding a "Politischer Arbeiterzirkel" ("political workers' union"), an order he carried out together with Anton Drexler in October 1918. On January 5, 1919, the DAP was formed, in which not only Harrer and Drexler but also Gottfried Feder and Dietrich Eckart were involved.