Muʿtazilah (Arabic: المعتزلة‎) is an Islamic school of speculative theology that flourished in the cities of Basra and Baghdad, both in present-day Iraq, during the 8th–10th centuries. The adherents of the Mu'tazili school are best known for their having asserted that, because of the perfect unity and eternal nature of Allah, the Qur'an must therefore have been created, as it could not be co-eternal with God. From this premise, the Mu'tazili school of Kalam proceeded to posit that the injunctions of God are accessible to rational thought and inquiry: because knowledge is derived from reason, reason is the "final arbiter" in distinguishing right from wrong. It follows, in Mu'tazili reasoning, that "sacred precedent" is not an effective means of determining what is just, as what is obligatory in religion is only obligatory "by virtue of reason."

The movement emerged in the Umayyad Era, and reached its height in the Abassid period. Scholarship on the movement stagnated for centuries owing to an absence of sympathetic accounts of the movement (and an abundance of hostile accounts) until the latter 20th century, when the 11th century texts of Abd al-Jabbar al-Qadi were unearthed in Yemen.