Thabit ibn Qurra abu' l'Hasan ibn Marwan al-Sabi al'Harrani, (826 – February 18, 901) was an Arab astronomer and mathematician. In Latin he was known as Thebit.

Thabit was born in Harran (antique Carrhae), Mesopotamia (now Turkey). At the invitation of Muhammad ibn Musa ibn Shakir , one of the Banu Musa brothers, Thabit went to study in Baghdad at the House of Wisdom. He belonged to the sect of the Harranian Sabians, often confused with the Mandaeans. As star-worshippers, Sabians showed a great interest in astronomy and mathematics. This sect lived in the vicinity of the main center of the Caliphate until 1258, when the Mongols destroyed their last shrine. During Muslim rule, they were a protected minority, and around the time of al-Mutawakkil's reign their town became a center for philosophical and medical learning. They were joined by the descendants of pagan Greek scholars who, having been persecuted in Europe, settled in lands that became part of the Abbasid caliphate. The Muslims were greatly interested in Greek culture and science, collecting and translating many ancient Greek works in the fields of philosophy and mathematics. Although they later became Arabic speakers, in pre-Islamic times, it was common for Sabians to speak Greek,

Thabit and his pupils lived in the midst of the most intellectually vibrant, and probably the largest, city of the time, Baghdad. He occupied himself with mathematics, astronomy, mechanics, medicine and philosophy. His native language was Syriac, which was the eastern Aramaic dialect from Edessa, and he knew Greek well too. He translated from Greek Apollonius, Archimedes, Euclid and Ptolemy. Thabit had revised translation of Euclid Elements of Hunayn ibn Ishaq. He had also rewritten Hunayn's translation of Ptolemy's Almagest and translated Ptolemy's Geography, which later became very well-known. Later in his life, Thabit's patron was the Abbasid Caliph al-Mu'tadid (reigned 892–902). Thabit became the Caliph's personal friend and courtier. Only a few of Thabit's works are preserved in their original form.

According to Copernicus Thabit determined the length of the sidereal year 365 days, 6 hours, 9 minutes and 12 seconds (an error of 2 seconds). He published his observations of the Sun. In mathematics Thabit discovered an equation for determining the amicable numbers.

He also wrote on the theory of numbers, and extended their use to describe the ratios between geometrical quantities, a step which The Greeks never took.

Thabit died in Baghdad. After him the greatest Sabean name was Abu Abdallah Mohammad ibn Jabir Al-Battani. Thabit and his grandson Ibrahim ibn Sinan ibn Thabit studied the curves needed for making sundials. Thabit's son Sinan ibn Thabit was a distinguished physician who was responsible for supervising all the public hospitals of Baghdad.