The Battle of Siffin (Arabic: ?????; May-July 657 CE) occurred during the First Fitna; or first Muslim civil war; with the main engagement taking place from July 26 to July 28. It was fought between Ali ibn Abi Talib and Muawiyah I; on the banks of the Euphrates river; in what is now Ar-Raqqah; Syria.

Following the controversial murder of Uthman ibn Affan; Ali had become Caliph but struggled to be accepted as such throughout the Muslim Empire. Muawiyah; the governor of Syria; was a kinsman of the murdered Caliph; and wanted the murderers brought to justice. He considered that Ali was unwilling to do this; and so Muawiyah rebelled against Ali; who attempted to put down the rebellion. The result was the engagement at Siffin. However; the battle was indecisive; and the two parties agreed to an arbitration; which was equally indecisive.

The battle and arbitration served to weaken Ali's position; but did not resolve the tensions that were plaguing the empire. To the Shia; Ali ibn Abi Talib was the first Imam. To Sunnis; Ali ibn Abi Talib was the fourth Rashidun Caliph; and Muawiyah the first Caliph of the Ummayyad dynasty. The events surrounding the battle are highly controversial between Sunni and Shia; and serve as part of the split between the two groups.