Umar ibn Abd al-Aziz (Arabic: عمر بن عبد العزيز, romanized: ʿUmar ibn ʿAbd al-ʿAzīz; 2 November 680 – c. 5 February 720), commonly known as Umar II (عمر الثاني), was the eighth Umayyad caliph. He made various significant contributions and reforms to the society, and he has been described as and was called the first Mujaddid and sixth righteous caliph of Islam after Hasan Bin Ali according to some Muslim Scholars.

He was also a cousin of the former caliph, being the son of Abd al-Malik's younger brother, Abd al-Aziz. He was also a matrilineal great-grandson of the second caliph, Umar ibn al-Khattab.

Surrounded with great scholars, he is credited with having ordered the first official collection of Hadiths and encouraged education to everyone. He also sent out emissaries to China and Tibet, inviting their rulers to accept Islam. At the same time, he remained tolerant with non-Muslim citizens. According to Nazeer Ahmed, it was during the time of Umar ibn Abd al-Aziz that the Islamic faith took roots and was accepted by huge segments of the population of Persia and Egypt.

Militarily, Umar is sometimes deemed a pacifist, since he ordered the withdrawal of the Muslim army in places such as Constantinople, Central Asia and Septimania despite being a good military leader. However, under his rule the Umayyads conquered many territories from the Christian kingdoms in the Iberian Peninsula.