An unlawful combatant, illegal combatant or unprivileged combatant/belligerent is a person who directly engages in armed conflict in violation of the laws of war and therefore is claimed not to be protected by the Geneva Conventions. The International Committee of the Red Cross points out that the terms "unlawful combatant", "illegal combatant" or "unprivileged combatant/belligerent" are not defined in any international agreements. While the concept of an unlawful combatant is included in the Third Geneva Convention, the phrase itself does not appear in the document. Article 4 of the Third Geneva Convention does describe categories under which a person may be entitled to prisoner of war status. There are other international treaties that deny lawful combatant status for mercenaries and children.

The Geneva Conventions apply in wars between two or more opposing sovereign states. They do not recognize any status of lawfulness for combatants in conflicts not involving two or more nation-states, such as during civil wars between state forces (territorial or third state) and non-state armed groups. A state in such a conflict is legally bound only to observe Common Article 3 of the Geneva Conventions. All parties are otherwise completely free to either apply or not apply any of the remaining Articles of the Conventions. Article 5 of the Third Geneva Convention states that the status of detainees whose combatant status is in doubt should be determined by a "competent tribunal". Until such time, they must be treated as prisoners of war. After a "competent tribunal" has determined that an individual is not a lawful combatant, the "detaining power" may choose to accord the individual the rights and privileges of a prisoner of war as described in the Third Geneva Convention, but is not required to do so. An individual who is not a lawful combatant, who is not a national of a neutral state living in the belligerent territory, and who is not a national of a co-belligerent state, retains rights and privileges under the Fourth Geneva Convention and must be "treated with humanity and, in case of trial, shall not be deprived of the rights of fair and regular trial".