Muawiyah I (Arabic: معاوية ابن أبي سفيان‎ Muʿāwiyah ibn ʾAbī Sufyān; 602 – 6 May 680) was the first Caliph of the Umayyad Dynasty. After the conquest of Mecca by the Muslims, Muawiyah's family converted to Islam. Muawiyah is brother-in-law to Muhammad who married his sister Ramlah bint Abi-Sufyan in 1AH. Muawiyah became a scribe for Muhammad, and during the first and second caliphates of Abu Bakr and Umar, fought with the Muslims against the Byzantines in Syria.

When Uthman ibn Affan, a cousin of Muawiyah, became the third caliph, he appointed Muawiyah Governor of Syria. However when Ali was appointed the fourth and final Rashidun Caliph, he expelled Muawiyah from the Governorship. Muawiyah refused to obey Ali, and had some level of support from the Syrians in his rebelliousness, amongst whom he was a popular leader. Ali called for military action against Muawiyah, but the reaction of the political classes in Medina was not encouraging, and thus Ali deferred. Eventually Ali marched on Damascus and fought Muawiyah's supporters at the inconclusive Battle of Siffin (657 CE).

Ali's son Hasan ibn Ali signed a truce and retired to private life in Medina. Muawiyah thus established the Umayyad Caliphate, which was to be a hereditary dynasty, and governed from Damascus in Syria instead of Medina in Arabia.

Muawiyah I is a reviled figure in Shia Islam for several reasons. Firstly, because of his involvement in the Battle of Siffin against Ali, whom the Shia Muslims believe was Muhammad's true successor (see Succession to Muhammad); secondly, for the alleged breaking of the treaty he made with Hasan ibn Ali, after the death of Hassan ibn Ali, by appointing his son Yazid as his successor; thirdly, on account of his responsibility for the killing of Hasan ibn Ali by alluring his wife Ja'dah binte Ash'as to poison him; and fourthly, for the deaths of various Companions of Muhammad.