Mu'awiya I (c. 597, 603 or 605–April 680) was the founder and first caliph of the Umayyad Caliphate, ruling from 661 until his death. He became caliph less than thirty years after the death of the Islamic prophet Muhammad and immediately after the four Rashidun ('rightly-guided') caliphs. Unlike his predecessors, who had been close, early companions of Muhammad, Mu'awiya was a relatively late follower of Muhammad.

Mu'awiya and his father Abu Sufyan had opposed Muhammad, their distant Qurayshite kinsman and later Mu'awiya's brother-in-law, until Muhammad captured Mecca in 630. Afterward, Mu'awiya became one of Muhammad's scribes. He was appointed by Caliph Abu Bakr (r. 632–634) as a deputy commander in the conquest of Syria. He moved up the ranks through Umar's caliphate (r. 634–644) until becoming governor of Syria during the reign of his Umayyad kinsman, Caliph Uthman (r. 644–656). He allied with the province's powerful Banu Kalb tribe, developed the defenses of its coastal cities, and directed the war effort against the Byzantine Empire, including the first Muslim naval campaigns. In response to Uthman's assassination in 656, Mu'awiya took up the cause of avenging the murdered caliph and opposed the election of Ali. During the First Muslim Civil War, the two led their armies to a stalemate at the Battle of Siffin in 657, prompting an abortive series of arbitration talks to settle the dispute. Afterward, Mu'awiya gained recognition as caliph by his Syrian supporters and his ally Amr ibn al-As, who conquered Egypt from Ali's governor in 658. Following the assassination of Ali in 661, Mu'awiya compelled Ali's son and successor Hasan to abdicate and Mu'awiya's suzerainty was acknowledged throughout the Caliphate.

Domestically, Mu'awiya relied on loyalist Syrian Arab tribes and Syria's Christian-dominated bureaucracy. He is credited with establishing government departments responsible for the postal route, correspondence, and chancellery. He was the first caliph whose name appeared on coins, inscriptions, or documents of the nascent Islamic empire. Externally, he engaged his troops in almost yearly land and sea raids against the Byzantines, including a failed siege of Constantinople. In Iraq and the eastern provinces, he delegated authority to the powerful governors al-Mughira and Ziyad ibn Abi Sufyan, the latter of whom he controversially adopted as his brother. Under Mu'awiya's direction, the Muslim conquest of Ifriqiya (central North Africa) was launched by the commander Uqba ibn Nafi in 670, while the conquests in Khurasan and Sijistan on the eastern frontier were resumed.

Although Mu'awiya confined the influence of his Umayyad clan to the governorship of Medina, he nominated his own son, Yazid I, as his successor. It was an unprecedented move in Islamic politics and opposition to it by prominent Muslim leaders, including Ali's son Husayn, and Abd Allah ibn al-Zubayr, persisted after Mu'awiya's death, culminating with the outbreak of the Second Muslim Civil War. While there is considerable admiration for Mu'awiya in the contemporary sources, he has been criticized for lacking the justice and piety of the Rashidun and transforming the office of the caliphate into a kingship. Besides these criticisms, Sunni Muslim tradition honors him as a companion of Muhammad and a scribe of Qur'anic revelation. In Shia Islam, Mu'awiya is reviled for opposing Ali, accused of poisoning his son Hasan, and held to have accepted Islam without conviction.

Mu'awiya's year of birth is uncertain, with 597, 603 or 605 cited by early Islamic sources. His father Abu Sufyan ibn Harb was a prominent Meccan merchant who led trade caravans to Syria, then part of the Byzantine Empire. He emerged as the leader of the Banu Abd Shams clan of the polytheistic Quraysh, the dominant tribe of Mecca, during the early stages of the Quraysh's conflict with Muhammad. The latter also hailed from the Quraysh and was distantly related to Mu'awiya via their common paternal ancestor, Abd Manaf ibn Qusayy. Mu'awiya's mother, Hind bint Utba, was also a member of the Banu Abd Shams.

In 624, Muhammad and his followers attempted to intercept a Meccan caravan led by Mu'awiya's father on its return from Syria, prompting Abu Sufyan to call for reinforcements. The Qurayshite relief army was routed in the ensuing Battle of Badr, in which Mu'awiya's elder brother Hanzala and their maternal grandfather, Utbah ibn Rabi'ah, were killed. Abu Sufyan replaced the slain leader of the Meccan army, Abu Jahl, and led the Meccans to victory against the Muslims at the Battle of Uhud in 625. After his abortive siege of Muhammad in Medina at the Battle of the Trench in 627, he lost his leadership position among the Quraysh.

Mu'awiya's father was not a participant in the truce negotiations at Hudaybiyya between the Quraysh and Muhammad in 628. The following year, Muhammad married Mu'awiya's widowed sister Umm Habiba, who had embraced Islam fifteen years earlier. The marriage may have reduced Abu Sufyan's hostility toward Muhammad and Abu Sufyan negotiated with him in Medina in 630 after confederates of the Quraysh violated the Hudaybiyya truce. When Muhammad captured Mecca in 630, Mu'awiya, his father, and his elder brother Yazid embraced Islam. According to accounts cited by the early Muslim historians al-Baladhuri and Ibn Hajar, Mu'awiya had secretly become a Muslim from the time of the Hudaybiyya negotiations. By 632 Muslim authority extended across Arabia with Medina as the seat of the Muslim government. As part of Muhammad's efforts to reconcile with the Quraysh, Mu'awiya was made one of his kātibs (scribes), being one of seventeen literate members of the Quraysh at that time. Abu Sufyan moved to Medina to maintain his newfound influence in the nascent Muslim community.

After Muhammad died in 632, Abu Bakr became caliph (leader of the Muslim community). He and his successors Umar, Uthman, and Ali are often known as the Rashidun ('rightly-guided') caliphs to distinguish them from Mu'awiya and his Umayyad dynastic successors. Having to contend with challenges to his leadership from the Ansar, the natives of Medina who had provided Muhammad safe haven from his erstwhile Meccan opponents, and the mass defections of several Arab tribes, Abu Bakr reached out to the Quraysh, particularly its two strongest clans, the Banu Makhzum and Banu Abd Shams, to shore up support for the Caliphate. Among those Qurayshites whom he appointed to suppress the rebel Arab tribes during the Ridda wars (632–633) was Mu'awiya's brother Yazid. Afterward, he was dispatched as one of four commanders in charge of the Muslim conquest of Byzantine Syria in c. 634. The caliph appointed Mu'awiya commander of Yazid's vanguard. Through these appointments Abu Bakr gave the family of Abu Sufyan a stake in the conquest of Syria, where Abu Sufyan already owned property in the vicinity of Damascus.