Marwan ibn al-Hakam ibn Abi al-As ibn Umayya (Arabic: مروان بن الحكم بن أبي العاص بن أمية, romanized: Marwān ibn al-Ḥakam ibn Abī al-ʿĀṣ ibn Umayya; 623 or 626 – April/May 685), commonly known as Marwan I, was the fourth Umayyad caliph, ruling for less than a year in 684–685. He founded the Marwanid ruling house of the Umayyad dynasty, which replaced the Sufyanid house after its collapse in the Second Fitna and remained in power until 750.

During the reign of his cousin, the third Rashidun caliph Uthman (r. 644–656), Marwan took part in a military campaign against the Byzantine Exarchate of Africa (in central North Africa), where he acquired significant war spoils. He also served as Uthman's governor in Fars (southwestern Iran) before becoming the caliph's katib (secretary or scribe). He was wounded fighting the rebel siege of Uthman's house, in which the caliph was slain. In the ensuing civil war between the fourth Rashidun caliph Ali (r. 656–661) and the largely Qurayshite partisans of Aisha, Marwan sided with the latter at the Battle of the Camel. Marwan later served as governor of Medina under his distant kinsman Caliph Mu'awiya I (r. 661–680), founder of the Umayyad Caliphate. During the reign of Mu'awiya's son and successor Yazid I (r. 680–683), Marwan organized the defense of the Umayyad realm in the Hejaz (western Arabia) against the local opposition which included prominent companions as well as Muhammad’s own clan, the Bani Hashim, who revolted under the banner of Muhammad’s grandson, Husayn ibn Ali. After Yazid died in November 683, the Mecca-based rebel and sahabi Abd Allah ibn al-Zubayr declared himself caliph and expelled Marwan, who took refuge in Syria, the center of Umayyad rule. With the death of the last Sufyanid caliph Mu'awiya II in 684, Marwan, encouraged by the ex-governor of Iraq Ubayd Allah ibn Ziyad, volunteered his candidacy for the caliphate during a summit of pro-Umayyad tribes in Jabiya. The tribal nobility, led by Ibn Bahdal of the Banu Kalb, elected Marwan and together they defeated the pro-Zubayrid Qays tribes at the Battle of Marj Rahit in August of that year.

In the months that followed, Marwan reasserted Umayyad rule over Egypt, Palestine, and northern Syria, whose governors had defected to Ibn al-Zubayr's cause, while keeping the Qays in check in the Jazira (Upper Mesopotamia). He dispatched an expedition led by Ibn Ziyad to reconquer Zubayrid Iraq, but died while it was underway in the spring of 685. Before his death, Marwan firmly established his sons in positions of power: Abd al-Malik was designated his successor, Abd al-Aziz was made governor of Egypt, and Muhammad oversaw military command in Upper Mesopotamia. Although Marwan was stigmatized as an outlaw and a father of tyrants in later anti-Umayyad tradition, the historian Clifford E. Bosworth asserts that the caliph was a shrewd, capable, and decisive military leader and statesman who laid the foundations of continued Umayyad rule for a further sixty-five years.

Marwan was born in 2 or 4 AH (623 or 626 CE). His father was al-Hakam ibn Abi al-As of the Banu Umayya (Umayyads), the strongest clan of the Quraysh, a polytheistic tribe which dominated the town of Mecca in the Hejaz. The Quraysh converted to Islam en masse in c. 630 following the conquest of Mecca by the Islamic prophet Muhammad, himself a member of the Quraysh. Marwan knew Muhammad and is thus counted among the latter's companions. Marwan's mother was Amina bint Alqama of the Kinana, the ancestral tribe of the Quraysh which dominated the area stretching southwest from Mecca to the Tihama coastline.

Marwan had at least sixteen children, among them at least twelve sons from five wives and an umm walad (concubine). From his wife A'isha, a daughter of his paternal first cousin Mu'awiya ibn Abu Sufyan, he had his eldest son Abd al-Malik, Mu'awiya and daughter Umm Amr. Umm Amr later married Sa'id ibn Khalid ibn Amr, a great-grandson of Marwan's paternal first cousin Uthman ibn Affan, who became the third caliph (leader of the Muslim community) in 644. Marwan's wife Layla bint Zabban ibn al-Asbagh of the Banu Kalb tribe bore him Abd al-Aziz and daughter Umm Uthman, who was married to Caliph Uthman's son al-Walid; al-Walid was also married at one point to Marwan's daughter Umm Amr. Another of Marwan's wives, Qutayya bint Bishr of the Banu Kilab, bore him Bishr and Abd al-Rahman, the latter of whom died young. One of Marwan's wives, Umm Aban al-Kubra, was a daughter of Caliph Uthman. She was mother to six of his sons, Aban, Uthman, Ubayd Allah, Ayyub, Dawud and Abd Allah, though the last of them died a child. Marwan was married to Zaynab bint Umar, a granddaughter of Abu Salama from the Banu Makhzum, who mothered his son Umar. Marwan's umm walad was also named Zaynab and gave birth to his son Muhammad. Marwan had ten brothers and was the paternal uncle of ten nephews.

During the reign of Caliph Uthman (r. 644–656), Marwan took part in a military campaign against the Byzantines of the Exarchate of Carthage (in central North Africa), where he acquired significant war spoils. These likely formed the basis of Marwan's substantial wealth, part of which he invested in properties in Medina, the capital of the Caliphate. At an undetermined point, he served as Uthman's governor in Fars (southwestern Iran) before becoming the caliph's katib (secretary or scribe) and possibly the overseer of Medina's treasury. According to the historian Clifford E. Bosworth, in this capacity Marwan "doubtless helped" in the revision "of what became the canonical text of the Qur'an" in Uthman's reign.

The historian Hugh Kennedy asserts that Marwan was the caliph's "right-hand man". According to some Muslim reports, many of Uthman's erstwhile backers among the Quraysh gradually withdrew their support as a result of Marwan's pervasive influence, which they blamed for the caliph's controversial decisions. The historian Fred Donner questions the veracity of these reports, citing the unlikelihood that Uthman would be so influenced by a younger relative such as Marwan and the rarity of specific charges against the latter, and describes them as a possible "attempt by later Islamic tradition to salvage Uthman's reputation as one of the so-called 'rightly-guided' (rāshidūn) caliphs by making Marwan the fall guy for the unhappy events at the end of Uthman's twelve-year reign."

Discontent over Uthman's nepotistic policies and confiscation of the former Sasanian crown lands in Iraq drove the Quraysh and the dispossessed elites of Kufa and Egypt to oppose the caliph. In early 656, rebels from Egypt and Kufa entered Medina to press Uthman to reverse his policies. Marwan recommended a violent response against them. Instead, Uthman entered into a settlement with the Egyptians, the largest and most outspoken group among the mutineers. On their return to Egypt, the rebels intercepted a letter in Uthman's name to Egypt's governor, Ibn Abi Sarh, instructing him to take action against the rebels. In reaction, the Egyptians marched back to Medina and besieged Uthman in his home in June 656. Uthman claimed to have been unaware of the letter, and it may have been authored by Marwan without Uthman's knowledge. Despite orders to the contrary, Marwan actively defended Uthman's house and was badly wounded in the neck when he challenged the rebels assembled at its entrance. According to tradition, he was saved by the intervention of his wet nurse, Fatima bint Aws, and was transported to the safety of her home by his mawla (freedman or client), Abu Hafs al-Yamani. Shortly after, Uthman was assassinated by the rebels, which became one of the major contributing factors to the First Fitna. After the assassination, Marwan and other Umayyads fled to Mecca. Calls for avenging Uthman's death were led by the Umayyads, one of Muhammad's wives, A'isha, and two of his prominent companions, al-Zubayr ibn al-Awwam and Talha ibn Ubayd Allah. Punishing Uthman's murderers became a rallying cry of the opposition to his successor, the fourth caliph Ali ibn Abi Talib, a cousin and son-in-law of Muhammad.