John Friedrich Karl Rittmeister, often also abbreviated John F. Rittmeister, was a German neurologist, psychoanalyst and resistance fighter against Nazism. Rittmeister was a humanist and socialist who based his opposition to the Nazi state on moral grounds. He was known as a communist member of the anti-fascist resistance group that was later called the Red Orchestra by the Abwehr.

The Red Orchestra, as it was known in Germany, was the name given by the Abwehr Section III.F to anti-Nazi resistance workers in August 1941. It primarily referred to a loose network of resistance groups, connected through personal contacts, uniting hundreds of opponents of the Nazi regime. These included groups of friends who held discussions that were centred on Harro Schulze-Boysen, Adam Kuckhoff and Arvid Harnack in Berlin, alongside many others. They printed and distributed prohibited leaflets, posters, and stickers, hoping to incite civil disobedience. They aided Jews and resistance to escape the regime, documented the atrocities of the Nazis, and transmitted military intelligence to the Allies. Contrary to legend, the Red Orchestra was neither directed by Soviet communists nor under a single leadership. It was a network of groups and individuals, often operating independently. To date, about 400 members are known by name.