A religious war (Latin: bellum sacrum) is a war caused by, or justified by, religious differences. It can involve one state with an established religion against another state with a different religion or a different sect within the same religion, or a religiously motivated group attempting to spread its faith by violence, or to suppress another group because of its religious beliefs or practices. The Crusades, the French Wars of Religion, the Muslim conquests, and the Reconquista are frequently cited historical examples.

In their Encyclopedia of Wars, authors Charles Phillips and Alan Axelrod attempt a comprehensive listing of wars in history. They document 1763 wars overall, of which 123 (7%) have been classified to involve a religious conflict. William T. Cavanaugh argues that what is termed "religious wars" is a largely Western dichotomy of different power configurations which serves a Western consumer audience.

Many wars that are considered religious wars have economic or political ramifications (land acquisition, control of trade routes, dynasty changes, etc.) that could call into question the true reasons behind the conflict. Differences in religion can further inflame a war being fought for other reasons. Historically, places of worship have been destroyed to weaken the morale of the opponent, even when the war itself is not being waged over religious ideals.^{[original research?]}