A religious war or a war of religion, sometimes also known as a holy war (Latin: sanctum bellum), is a war which is primarily caused or justified by differences in religion. In the modern period, there are frequent debates over the extent to which religious, economic, ethnic or other aspects of a conflict are predominant in a given war. The degree to which a war may be considered religious depends on many underlying questions, such as the definition of religion, the definition of 'religious war' (taking religious traditions on violence such as 'holy war' into account), and the applicability of religion to war as opposed to other possible factors. Answers to these questions heavily influence conclusions on how prevalent religious wars have been as opposed to other types of wars.

According to scholars such as Jeffrey Burton Russell, conflicts may not be rooted strictly in religion and instead may be a cover for the underlying secular power, ethnic, social, political, and economic reasons for conflict. Other scholars have argued that what is termed "religious wars" is a largely "Western dichotomy" and a modern invention from the past few centuries, arguing that all wars that are classed as "religious" have secular (economic or political) ramifications. In several conflicts including the Israeli–Palestinian conflict, the Syrian civil war, and the wars in Afghanistan and Iraq, religious elements are overtly present, but variously described as fundamentalism or religious extremism—depending upon the observer's sympathies. However, studies on these cases often conclude that ethnic animosities drive much of the conflicts.

According to the Encyclopedia of Wars, out of all 1,763 known/recorded historical conflicts, 121, or 6.87%, had religion as their primary cause. Matthew White's The Great Big Book of Horrible Things gives religion as the primary cause of 11 of the world's 100 deadliest atrocities.