Preventive detention is an imprisonment that is putatively justified for non-punitive purposes, most often to prevent (further) criminal acts.

There is no universally agreed definition of preventive detention, and multiple types of detention are sometimes considered a form of preventive detention.

Usually, preventive detention is the detention of a convicted criminal who has served their sentence, but is considered too dangerous to release. In that case, the detention is considered "preventive" in that it is not intended to punish or deter the criminal, but to prevent the criminal from committing further crimes and/or also protect the public.

This article focuses on both this type of preventive detention and detention of a person without trial or conviction by a court. Its purpose is to not to punish a person for their past offences, but to prevent him from committing an offence in the near future.