Hunayn ibn Ishaq al-Ibadi (808–873; also Hunain or Hunein; Arabic: أبو زيد حنين بن إسحاق العبادي; ʾAbū Zayd Ḥunayn ibn ʾIsḥāq al-ʿIbādī; known in Latin as Johannitius) was an influential Arab Nestorian Christian translator, scholar, physician, and scientist. During the apex of the Islamic Abbasid era, he worked with a group of translators, among whom were Abū 'Uthmān al-Dimashqi, Ibn Mūsā al-Nawbakhti, and Thābit ibn Qurra, to translate books of philosophy and classical Greek and Persian texts into Arabic and Syriac.

Ḥunayn ibn Isḥaq was his era's most productive translator of Greek medical and scientific treatises. He studied Greek and became known as the "Sheikh of the Translators". He mastered four languages: Arabic, Syriac, Greek and Persian. Later translators widely followed Hunayn's method. He was originally from al-Hirah, previously the capital of the Lakhmid kingdom, but worked in Baghdad, the center of the Translation movement. His fame went far beyond the local community.

In the Abbasid era, a new interest in extending the study of Greek science had arisen. At that time, there was a vast amount of untranslated ancient Greek literature pertaining to philosophy, mathematics, natural science, and medicine. This valuable information was only accessible to a very small minority of Middle Eastern scholars who knew the Greek language; the need for an organized translation movement was urgent.

In time, Hunayn ibn Ishaq became arguably the chief translator of the era, and laid the foundations of Islamic medicine. In his lifetime, ibn Ishaq translated countless works, including Plato's Timaeus, Aristotle's Metaphysics, and the Old Testament, into Syriac and Arabic. He personally translated 129 works of Galen himself, even travelling to find one, De demonstratione which he found half of in Damascus. Ibn Ishaq also produced 36 of his own books, 21 of which covered the field of medicine. His son Ishaq, and his nephew Hubaysh, worked together with him at times to help translate. Hunayn ibn Ishaq is known for his translations, his method of translation, and his contributions to medicine. He has also been suggested by François Viré to be the true identity of the Arabic falconer Moamyn, author of De Scientia Venandi per Aves. Hunayn Ibn Ishaq was a translator at the House of Wisdom, Bayt al-Hikma, where he received his education. In the West, another name he is known by his Latin name, Joannitius. It was the translations that came from administrative and legal materials gathered that lead to understanding of how to build up Arabic as the new official language.

Hunayn ibn Ishaq was a Syriac Christian and Church of the East follower, born in 808, during the Abbasid period, in al-Hirah, to an ethnic Syriac family. Hunayn in classical sources is said to have belonged to the ʿIbad, thus his nisba "al-Ibadi. The ʿIbad was an Arab community composed of different Arab tribes that had once converted to Nestorian Christianity and lived in al-Hira. They were known for their literacy and multilingualism, being fluent in Syriac, their liturgical and cultural language, in addition to their native Arabic.

As a child, he learned the Syriac and Arabic languages. Although al-Hira was known for commerce and banking, and his father was a pharmacist, Hunayn went to Baghdad in order to study medicine. In Baghdad, Hunayn had the privilege to study under renowned physician Yuhanna ibn Masawayh; however, Hunayn's countless questions irritated Yuhanna, causing him to scold Hunayn and forcing him to leave. Hunayn promised himself to return to Baghdad when he became a physician. He went abroad to master the Greek language. On his return to Baghdad, Hunayn displayed his newly acquired skills by reciting the works of Homer and Galen. In awe, ibn Masawayh reconciled with Hunayn, and the two started to work cooperatively.

Hunayn was extremely motivated in his work to master Greek studies, which enabled him to translate Greek texts into Syriac and Arabic. The Abbasid Caliph al-Mamun noticed Hunayn's talents and placed him in charge of the House of Wisdom, the Bayt al Hikmah. The House of Wisdom was an institution where Greek works were translated and made available to scholars. (At least one scholar has argued that there is no evidence of Hunayn being placed in charge of the Bayt al Hikmah.) The caliph also gave Hunayn the opportunity to travel to Byzantium in search of additional manuscripts, such as those of Aristotle and other prominent authors.

In Hunayn ibn Ishaq's lifetime, he devoted himself to working on a multitude of writings; both translations and original works.