An auxiliary force is a group affiliated with, but not part of, a military or police organization. In some cases, auxiliaries are armed forces operating in the same manner as regular soldiers. Most often, auxiliaries are civilian volunteer components supporting the main police or military force.

Historically the designation "auxiliary" has also been given to foreign or allied troops in the service of a nation at war.

The Auxiliary Division was a British paramilitary police force raised during the Irish War of Independence 1919–21. Colonial Auxiliary Forces were used in Africa during the Anglo-Boer War and elsewhere.

German paramilitary police forces, called Hilfspolizei or Schutzmannschaft, were raised during World War II and were the collaborationist auxiliary police battalions of native policemen in occupied countries, which were created to fight the resistance during World War II mostly in the Eastern European countries occupied by Nazi Germany. Hilfspolizei refers also to German auxiliary police units. There was also a HIPO Corps in occupied Denmark. The term is mostly historical; it had also been applied to some units created in 1933 by the early Nazi government (mostly from members of SA and SS) and disbanded the same year due to international protests.