Military justice (also military law) is the legal system (bodies of law and procedure) that governs the conduct of the active-duty personnel of the armed forces of a country. In some nation-states, civil law and military law are distinct bodies of law, which respectively govern the conduct of civil society and the conduct of the armed forces; each body of law has specific judicial procedures to enforce the law. Among the legal questions unique to a system of military justice are the practical preservation of good order and discipline, command responsibility, the legality of orders, war-time observation of the code of conduct, and matters of legal precedence concerning civil or military jurisdiction over the civil offenses and the criminal offenses committed by active-duty military personnel.

Military justice is different and distinct from martial law, which is the imposition of direct military authority upon a civilian population, in place of the civilian legal system of law that authorised the government to rule. Martial law is declared in times of emergency, civil unrest, and war, but its imposition can be restricted or even banned by civil law.