A kapo or prisoner functionary (German: Funktionshäftling) was a prisoner in a Nazi camp who was assigned by the Schutzstaffel (SS) guards to supervise forced labor or carry out administrative tasks.

Also called "prisoner self-administration", the prisoner functionary system minimized costs by allowing camps to function with fewer SS personnel. The system was designed to turn victim against victim, as the prisoner functionaries were pitted against their fellow prisoners in order to maintain the favor of their SS overseers. If they neglected their duties, they would be demoted to ordinary prisoners and be subject to other kapos. Many prisoner functionaries were recruited from the ranks of violent criminal gangs rather than from the more numerous political, religious, and racial prisoners; such criminal convicts were known for their brutality toward other prisoners. This brutality was tolerated by the SS and was an integral part of the camp system.

Prisoner functionaries were spared physical abuse and hard labor, provided they performed their duties to the satisfaction of the SS functionaries. They also had access to certain privileges, such as civilian clothes and a private room. While the Germans commonly called them kapos, the official government term for prisoner functionaries was Funktionshäftling.

After World War II, the term was reused as an insult; according to The Jewish Chronicle, it is "the worst insult a Jew can give another Jew".