The Basij (Persian: بسيج‎) (officially Basij-e Mostaz'afin, literally "Mobilization of the Oppressed") is a paramilitary volunteer militia established in 1979 by order of the Islamic Revolution's leader Ayatollah Khomeini. The original organization comprised the civilian volunteers whom the Ayatollah Khomeini urged to fight in the Iran-Iraq war. The force consists of young Iranians who have volunteered, often in exchange for official benefits. Currently Basij serve as an auxiliary force engaged in activities such as internal security as well as law enforcement auxiliary, the providing of social service, organizing of public religious ceremonies, and, more notoriously, policing of morals and the suppression of dissident gatherings.Basij is the name of the force; a basiji is an individual member.

The Basij are set up as subordinate to, receiving their orders from, the Iranian Revolutionary Guards and the current Supreme Leader Ayatollah Khamenei to whom they are known for their loyalty. They have also been described as "a loosely allied group of organizations" including "many groups controlled by local clerics." They have a local organization in almost every city in Iran.