Grand Ayatollah Muhammad Husayn Fadlallah (Arabic: محمد حسين فضل الله, romanized: Muḥammad Ḥusayn Fadl Allāh; 16 November 1935 – 4 July 2010) was a prominent Lebanese-Iraqi Twelver Shia cleric. Born in Najaf, Iraq, Fadlallah studied Islam in Najaf before moving to Lebanon in 1952. In the following decades, he gave many lectures, engaged in intense scholarship, wrote dozens of books, founded several Islamic religious schools, and established the Mabarrat Association. Through the aforementioned association, he established a public library, a women's cultural center, and a medical clinic.

Fadlallah was sometimes called the "spiritual mentor" of Hezbollah in the media, and is the Marja' of Naim Qassem, although his mentorship of Hezbollah was disputed by other sources. He was also the target of several assassination attempts, including the 1985 Beirut car bombing.

His death was followed by a huge turnout in Lebanon, visits by virtually all major political figures across the Lebanese spectrum, and statements of condolence from across the greater Middle East region; but it also led to controversy in the West and a denunciation in Israel.

Fadlallah was born in the Iraqi Shia shrine city of Najaf on 16 November 1935. His parents, Abdulraouf Fadlullah and al-Hajja Raoufa Hassan Bazzi, had migrated there from the village of 'Aynata in south Lebanon in 1928 to learn theology. By the time of his birth, his father was already a Muslim scholar.

Fadlallah began his education starting with learning the Quran and the alphabet in seminary classes in a Quranic Kuttab School in Najaf, including basic skills of writing and reading. He later on began in an Islamic School in Najaf called Muntada al-Nashir which was recently formed at that time.

Poetry gave great inspiration to Fadlallah from magazines such as al-Katib published in Egypt and already from a teenager, he began to write poetry himself. He also showed critical views towards the occupation of Palestine in 1947 by publishing a poem for the public by the age of 12. By that time, he had begun his religious studies in a Hawza school. However, the Hawza school in Najaf focused exclusively on legal studies and his poetry was not well received. This did not stop him from continuing writing poetry and Najaf had a guiding school for poets called Muntada al-Nashir which he joined later on. He also got support from his uncle who was also a poet himself and meant that poetry was important to achieve greater understanding of Islam.

After 21 years of studying under the prominent teachers of the Najaf religious university he concluded his studies in 1966 and returned to Lebanon. He had already visited Lebanon in 1952 where he recited a poem eulogizing Muhsin al Amin at his funeral.

In 1966 Fadlallah received an invitation from a group who had established a society called "The family of Fraternity" (جمعية أسرة التآخي Jam'iyat Usrat at-Ta'akhi) to come and live with them in the area of Naba'a in Eastern Beirut. He agreed, especially as the conditions at Najaf impelled him to leave.