Khajeh Nasir ad-Din Toosi (1201–1274) was a Persian scientist, of Shi'a Twelver Islamic belief, born in Toos, Khorasan, Iran. He is known as a philosopher, mathematician, astronomer, theologian, physician, and a prolific writer, i.e., he was a polymath.

Nasir ad-Din Toosi was born in Toos in the year 1201 and began his studies at an early age. In Toosi he studied Arabic, the Qur'an, Hadith, Shi'a Jurisprudence, logic, philosophy, mathematics, medicine and astronomy.

At a young age he moved to Nishapur to study Philosophy under Farid al-Din Damad and Mathematics under Muhammad Hasib.

As the armies of Genghis Khan swept his homeland, he fled to join the Ismailis and made his most important contributions in science during this time, while he was moving from one stronghold to another. He finally joined Hulagu's (Genghis Khan's son) ranks, after the invasion of the Alamut castle of the Hashshashin (Assassins) by Mongol forces.

The Mongol ruler, Hulagu Khan, built an observatory in Maragheh for Toosi. Toosi is considered the first of the Maragheh School of astronomers, who made significant contributions to the development of astronomy. Notable members of that school were Mu’ayyad al-Din al-’Urdi and Qutb al-Din al-Shirazi.

Toosi made very accurate tables of planetary movements as depicted in his book Zij-i ilkhani (the Ilkhanic Tables). This books contains astronomical tables for calculating the positions of the planets and the names of the stars. His planetary system was the most advanced of his period and was used extensively until the development of the heliocentric model in the time of Copernicus. Between Ptolemy and Copernicus, he is considered as the most eminent astronomer of his time.

He was perhaps the first to treat trigonometry as a separate mathematical discipline and in his Treatise on the Quadrilateral he was the first to list the six distinct cases of a right angled triangle in spherical trigonometry.

He invented the geometrical technique called a Toosi-couple for his planetary models, which generates linear motion from the sum of two circular motions. He also calculated the value of 51" for the annual precession of the equinoxes and contributed to construction and usage of some astronomical instruments including astrolabe. He gave the first extant exposition of the whole system of plane and spherical trigonometry.

A 60-km diameter lunar crater located on the southern hemisphere of the moon is named after him as "Nasireddin".

He also wrote extensively on biology and is one of the early pioneers of a kind of evolutionism in scientific thought.