An affinity group is a small group of activists (usually from 3-20) who work together on direct action.

Affinity groups are organized in a non-hierarchical manner, usually using consensus decision making, and are often made up of trusted friends or other like-minded people. They provide a method of organization that is responsive, flexible and decentralized.

Affinity groups can be based on a common ideology (eg. anarchism), a shared concern for a given issue (eg. anti-nuclear) or a common activity, role or skill (eg. street medics). Affinity groups may have either open or closed membership, although the latter is far more common.