ʿUmar ibn al-Khaṭṭāb (Arabic: عمر بن خطاب, romanized: ʿUmar bin Khaṭṭāb, also spelled Omar, c. 582/583 – 644) was the second Rashidun caliph, ruling from August 634 until his assassination in 644. He succeeded Abu Bakr (r. 632–634) as the second caliph of the Rashidun Caliphate on 23 August 634. Umar was a senior companion and father-in-law of the Islamic prophet Muhammad. He was also an expert Muslim jurist known for his pious and just nature, which earned him the epithet Al-Fārūq ("the one who distinguishes (between right and wrong)").

Umar initially opposed Muhammad, his distant Qurayshite kinsman and later son-in-law. Following his conversion to Islam in 616, he became the first Muslim to openly pray at the Kaaba. Umar participated in almost all battles and expeditions under Muhammad, who bestowed the title al-Fārūq (the Distinguisher) upon Umar, for his judgements. After Muhammad's death in June 632, Umar pledged allegiance to Abu Bakr (r. 632–634) as the first caliph and served as the closest adviser to the latter until August 634, when the dying Abu Bakr nominated Umar as his successor.

Under Umar, the caliphate expanded at an unprecedented rate, ruling the Sasanian Empire and more than two-thirds of the Byzantine Empire. His attacks against the Sasanian Empire resulted in the conquest of Persia in less than two years (642–644). According to Jewish tradition, Umar set aside the Christian ban on Jews and allowed them into Jerusalem and to worship. Umar was assassinated by the Persian slave Abu Lu'lu'a Firuz in 644.

Umar is generally viewed by historians to be one of the most powerful and influential Muslim caliphs in history. He is revered in the Sunni Islamic tradition as a great just ruler and paragon of Islamic virtues, and some hadiths identify him as the second greatest of the Sahabah after Abu Bakr. He is viewed negatively in the Twelver Shia tradition.