Sufism (Arabic: الصُّوفِيَّة‎ aṣ-ṣūfiyya), also known as Tasawwuf (التَّصَوُّف‎ at-taṣawwuf), is a mystic body of religious practice emerged from the teachings of Ali recited by Hasan al-Babri, today found not only within Sunni Islam but also within Shia Islam, which is characterized by a focus on Islamic purification, spirituality, ritualism, asceticism and esotericism. It has been variously defined as "Islamic mysticism", "the mystical expression of Islamic faith", "the inward dimension of Islam", "the phenomenon of mysticism within Islam", the "main manifestation and the most important and central crystallization" of mystical practice in Islam, and "the interiorization and intensification of Islamic faith and practice".

Practitioners of Sufism are referred to as "Sufis" (from صُوفِيّ, ṣūfīy), and historically typically belonged to "orders" known as tariqa (pl. ṭuruq) – congregations formed around a grand wali who would be the last in a chain of successive teachers linking back to Muhammad, with the goal of undergoing Tazkiah (self purification) and the hope of reaching Ihsan.

Sufism emerged early on in Islamic history, partly as a reaction against the worldliness of the early Umayyad Caliphate (661–750) and mainly under the tutelage of Hasan Al-Basri. Although Sufis were opposed to dry legalism, they strictly observed Islamic law and belonged to various schools of Islamic jurisprudence and theology. Although the overwhelming majority of Sufis, both pre-modern and modern, remain adherents of Sunni Islam, certain strands of Sufi thought transferred over to the ambits of Shia Islam during the late medieval period. This particularly happened after the Safavid conversion of Iran under the concept of Irfan. Important focuses of Sufi worship include dhikr, the practice of remembrance of God. Sufis also played an important role in spreading Islam through their missionary and educational activities.

Despite a relative decline of Sufi orders in the modern era and attacks from revivalist Islamic movement (such as the Salafis and Wahhabis), Sufism has continued to play an important role in the Islamic world, especially in the neo-traditionalist strand of Sunni Islam. It has also influenced various forms of spirituality in the West and generated lots of academic interest.