Deir ez-Zor (Arabic: دَيْرُ ٱلزَّوْرِ / دَيْرُ ٱلزُّور, romanized: Dayru z-Zawr / Dayru z-Zūr) is the largest city in eastern Syria and the seventh largest in the country. Located on the banks of the Euphrates 450 km (280 mi) to the northeast of the capital Damascus, Deir ez-Zor is the capital of the Deir ez-Zor Governorate. In the 2018 census, it had a population of 271,800.

Ad-Deir is a common shorthand for Deir ez-Zor. In the Syriac language of the Assyrian Christian population, Zeʿūrta (ܙܥܘܪܬܐ) means "little"; hence, Dīrā Zeʿūrta means "small habitation". The current name, which has been extended to the surrounding region, indicates an ancient site for one of the Early Christian secluded Syriac monasteries established during the persecution of Christians in the Roman Empire and Apostolic Age throughout Mesopotamia. Although Deir (ܕܝܪܐ), which is Arabic (borrowed from Syriac) for "monastery", is believed to have been kept throughout the various Medieval and modern age renamings, Zor, which indicates the riverbank bush, appeared only in some late Ottoman records of the Deir ez-Zor Vilayet.

Many different romanizations are used, including Deir Ezzor, Deir Al-Zor, Deir-al-Zour, Dayr Al-Zawr, Der Ezzor, Deir Azzor, Der Zor, and Deirazzor.

Archaeological findings in Deir ez-Zor indicate that the area has been inhabited since the ninth millennium BC. While the current location of the city has not always had a significant population, it was always an urban area, usually subordinate to more powerful Semitic cities, such as kingdoms like the Kingdom of Mari, which rose in the third millennium BC.

During the third millennium BC, the Amorites settled the area and established the kingdom of Yamhad, one of whose urban centers was the city of Deir Ez-Zor (alongside Mayadeen, Qars, and Tarka and its capital of Aleppo). The city didn't suffer during the succession of major empires (such as the Akkadian Empire, Old Assyrian Empire, Babylonian Empire, Hittite Empire, Middle Assyrian Empire and Neo-Assyrian Empire) when some military campaigns by the emperors were destroying entire urban centers for fear of future rebellion, as Deir al-Zour was too small to be considered a threat, and the region was incorporated into Assyria during the Iron Age.

In the third century BC, Alexander the Great crossed the region and built the city of Dura-Europos. Although influenced by Greek culture, the Semitic Aramaic language remained prevalent in the city. When Syria came under the Roman Empire in 64 BC, Deir Ez-Zor was a small, marginal village known as Azdra, which the Romans made the center of the region and founded a strong military garrison. Deir Ez-Zor came under the reign of Queen Zenobia of Palmyra in the third century, within an autonomous federation of the Roman Empire.

After the end of the Ridda wars in the Arabian Peninsula, Abu Bakr sent four armies to the Levant, led by Yazid ibn Abi Sufyan, Abu Ubaidah ibn al-Jarrah, Amr ibn al-Aas, and Shurahbil ibn Hasana. Because of the strength and size of the armies of the Byzantine Empire, Abu Bakr ordered Khalid ibn al-Walid to march with half of the Muslim Arab army to the Levant and command the armies there.

Khalid set off with his army towards Sham and opened Bosra and then defeated the Byzantines at the Battle of Ajnadayn. After Umar ibn Al-Khattab became caliph in 13 AH (634 AD), Khalid was replaced by Abu Ubaidah ibn al-Jarrah. Abu Ubaidah was ordered to complete the conquest. He took Damascus, Baalbek, Homs, Hama and Latakia.