Ali ibn Abi Talib (Arabic: عليّ بن أبي طالب, romanized: ʿAlī ibn Abī Ṭālib; c. 600 – 661 CE) was the fourth Rashidun caliph who ruled from 656 CE until his assassination in 661, as well as the first Shia Imam. He was the cousin and son-in-law of the Islamic prophet Muhammad. Born to Abu Talib ibn Abd al-Muttalib and Fatima bint Asad, Ali was raised in the household of his cousin Muhammad and was among the first to accept his teachings.

Ali played a pivotal role in the early years of Islam when Muslims were severely persecuted in Mecca.[citation needed] After the immigration (hijra) to Medina in 622, Muhammad gave his daughter Fatima to Ali in marriage and swore a pact of brotherhood with him. Ali served as Muhammad's secretary and deputy in this period, and was the flag-bearer of his army. Numerous sayings of Muhammad praise Ali, the most controversial of which was uttered in 632 at the Ghadir Khumm, Whoever I am his mawla, then Ali is his mawla. The interpretation of the polysemous Arabic word mawla is disputed: for Shia Muslims, Muhammad thus invested Ali with his religious and political authority, while Sunni Muslims view this as a mere statement of friendship and rapport. When Muhammad died in the same year, a group of Muslims met in the absence of Ali and appointed Abu Bakr (r. 632–634) as their leader. Ali later relinquished his claims to leadership and resigned from public life during the reigns of Abu Bakr and his successor, Umar (r. 634–644). Even though his advice was occasionally sought, the conflicts between Ali and the first two caliphs are epitomized by his refusal to follow their practices. This refusal cost Ali the caliphate to the benefit of Uthman (r. 644–656), who was thus appointed to succeed Umar by the electoral council. Ali was also highly critical of Uthman, who was widely accused of nepotism and corruption. Yet Ali also repeatedly mediated between the caliph and the provincial dissidents angered by his policies.

Following Uthman's assassination in June 656, Ali was elected caliph in Medina. He immediately faced two separate rebellions to avenge Uthman: The triumvirate of Talha, Zubayr, both companions of Muhammad, and his widow Aisha captured Basra in Iraq but were defeated by Ali in the Battle of the Camel in 656. Elsewhere, Mu'awiya, whom Ali had removed from the governorship of Syria, fought against Ali in the inconclusive Battle of Siffin in 657, which ended in a failed arbitration process that alienated some of Ali's supporters. These formed the Kharijites, who later terrorized the public and were crushed by Ali in the Battle of Nahrawan in 658. Ali was assassinated in 661 by the Kharijite dissident Ibn Muljam, which paved the way for Mu'awiya to seize power and found the dynastic Umayyad Caliphate.

Ali is revered for his courage, honesty, unbending devotion to Islam, magnanimity, and equal treatment of all Muslims. For his admirers, he has thus become the archetype of uncorrupted Islam and pre-Islamic chivalry. Sunni Muslims regard him as the last of the rashidun (lit. 'rightly-guided') caliphs, while Shia Muslims venerate him as their first Imam and the rightful religious and political successor to Muhammad. Ali's place is said to be second only to Muhammad in Shia Muslim culture. The shrine of Ali in Najaf, Iraq, is a major destination for Shia pilgrimage. The legacy of Ali is collected and studied in numerous books, the most famous of which is Nahj al-balagha.