In Shia Islam, marjiʿ (Arabic: مرجع‎; plural: marājiʿ), also known as a marjiʿ taqlīd or marjiʿ dīnī (Arabic: مرجع تقليد مرجع ديني‎), literally meaning "source to imitate/follow" or "religious reference", is a title given to the highest level Shia authority, a Grand Ayatollah with the authority to make legal decisions within the confines of Islamic law for followers and less-credentialed clerics. After the Qur'an and the prophets and imams, marājiʿ are the highest authority on religious laws in Usuli Shia Islam.

Currently, marājiʿ are accorded the title Grand Ayatollah (Arabic: آية ‌الله العظمی‎ Āyatullāh al-Uẓmà; Persian: آیت الله عظما‎ Ayatollah Ozma), however when referring to one, the use of Ayatollah is not acceptable. Previously, the titles of Allamah (such as Allameh Tabatabaei, Allameh Majlesi, Allameh Hilli) and Imam (such as Imam Khomeini, Imam Rohani,^[1] Imam Shirazi^[2] and Imam Sadr)^[3] have also been used.

From the perspective of jurisprudence, during the occultation of Muhammad al-Mahdi, Shiite clerics are his representatives, with responsibility for understanding and explaining Islamic religious jurisprudence.

Shiite authorities in the history of Shi'ism have an important role in the religious, political and social thought of their communities. One example is the fatwa of Mirza Mohammed Hassan Husseini Shirazi imposing sanctions on the use of tobacco during Qajar rule, which led to the abolition of the tobacco concession.^[4]

The shi' of an ayatollah transpires when he becomes a celebrated figure in the hawza and his students and followers trust him in answering their questions, and ask him to publish a juristic book, the risālah ʿamalīyah—a manual of practical rulings arranged according to topics dealing with ritual purity, worship, social issues, business, and political affairs. The risālah contains an ayatollah's fatwas on different topics, according to his knowledge of the most authentic Islamic sources and their application to current life. Traditionally only the most renowned ayatollahs of the given time published a risālah. Today, however, many ayatollahs of varying degrees of illustriousness have published one, while some of the renowned ones have refused to do so.

Where a difference in opinion exists between the marājiʿ, each of them provides their own opinion and the Muqallid will follow his/her own marjaʿ's opinion on that subject.^[5] A mujtahid, i.e. someone who has completed advanced training (dars kharij) in the hawza and has acquired the license to engage in ijtihad (ʾijāz al-ʾijtihād) from one or several ayatollahs, is exempted from the requirement to follow a marjaʿ. However ijtihad is not always comprehensive and so a mujtahid may be an expert in one particular area of Islamic jurisprudence (fiqh) and exercise ijtihad therein, but follow a marjaʿ in other areas of fiqh.

Several senior Grand Ayatollahs preside over hawzas, religious seminaries. The hawzas of Qom and Najaf are the preeminent seminary centers for the training of Shia clergymen. However, there are other smaller hawzas in other cities around the world, such as Karbala in Iraq, and Isfahan and Mashhad in Iran.

There are 86 Maraji living worldwide as of 2017,^[6][7] mainly in Najaf and Qom. The most prominent among them are Ali Khamenei in Tehran, Ali al-Sistani, Muhammad al-Fayadh, Muhammad Saeed al-Hakim and Bashir al-Najafi in Najaf; and Hossein Vahid Khorasani, Mousa Shubairi Zanjani, Sayyid Sadeq Rohani, Lotfollah Safi Golpaygani, Naser Makarem Shirazi, Sadiq Hussaini Shirazi, Hossein Noori Hamedani and Abdollah Javadi-Amoli in Qom.