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 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 government's forces, and insurgents. 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, 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".

In the United States, the Military Commissions Act of 2006 codified the legal definition of this term and invested the U.S. President with broad discretion to determine whether a person may be designated an unlawful enemy combatant under United States law.

The assumption that unlawful combatant status exists as a separate category to lawful combatant and civilian is contradicted by the findings of the International Criminal Tribunal for the Former Yugoslavia in the Celebici Judgment. The judgment quoted the 1958 International Committee of the Red Cross (ICRC) commentary on the Fourth Geneva Convention: "Every person in enemy hands must be either a prisoner of war and, as such, be covered by the Third Convention; or a civilian covered by the Fourth Convention. There is no intermediate status; nobody in enemy hands can be outside the law". Thus, anyone not entitled to prisoner of war status maintains the same rights as a civilian, and must be prosecuted under domestic law. Neither status exists in non-international conflict, with all parties equally protected under International Humanitarian Law.