An inquisitorial system is a legal system in which the court, or a part of the court, is actively involved in investigating the facts of the case. This is distinct from an adversarial system, in which the role of the court is primarily that of an impartial referee between the prosecution and the defense. Inquisitorial systems are used primarily in countries with civil legal systems, such as France and Italy, or legal systems based on Islamic law like Saudi Arabia, rather than in common law systems. It is the prevalent legal system in Continental Europe, Latin America, African countries not formerly under British rule, East Asia (except Hong Kong), Indochina, Thailand, the Philippines, and Indonesia. Most countries with an inquisitorial system also have some form of civil code as their main source of law.

Countries using common law, including the United States, may use an inquisitorial system for summary hearings in the case of misdemeanors or infractions, such as minor traffic violations. The distinction between an adversarial and inquisitorial system is theoretically unrelated to the distinction between a civil legal and common-law system. Some legal scholars consider inquisitorial misleading, and prefer the word nonadversarial. The function is often vested in the office of the public procurator, as in China, Japan, and Germany.