Harūn al-Rashīd (; Arabic: هَارُون الرَشِيد Hārūn Ar-Rašīd, "Aaron the Just" or "Aaron the Rightly-Guided"; 17 March 763 or February 766 – 24 March 809 CE / 148–193 AH) was the fifth Caliph of the Abbasid Dynasty. His birth date is debated, with various sources giving dates from 763 to 766. His epithet "al-Rashid" translates to "the Orthodox", "the Just", "the Upright", or "the Rightly-Guided". He ruled from 786 to 809, traditionally regarded to be the beginning of the Islamic Golden Age.

Harun established the legendary library Bayt al-Hikma ("House of Wisdom") in Baghdad in present-day Iraq, and during his rule Baghdad began to flourish as a world center of knowledge, culture and trade. During his rule, the family of Barmakids, which played a deciding role in establishing the Abbasid Caliphate, declined gradually. In 796, he moved his court and government to Raqqa in present-day Syria.

A Frankish mission came to offer Harun friendship in 799. Harun sent various presents with the emissaries on their return to Charlemagne's court, including a clock that Charlemagne and his retinue deemed to be a conjuration because of the sounds it emanated and the tricks it displayed every time an hour ticked. Portions of the fictional One Thousand and One Nights are set in Harun's court and some of its stories involve Harun himself. Harun's life and court have been the subject of many other tales, both factual and fictitious.