Nasir Tusi or Abu Jafar Muhammad Ibn Muhammad Ibn al-Hasan Nasir al-Din al-Tusi (1201–1274, near Baghdad) was a Persian scientist, of Shi'a islamic belief, born in Tus, Khorasan (then Persia, present time Iran). He is known to be a philosopher, mathematicians, astronomer, theologian, physician, and a prolific writer.

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 (Ghenghis Khans' son) ranks, after the invasion of the Alamut castle of the Hashshashin (Assassins) by Mongol forces.

Tusi made very accurate tables of planetary movements as depicted in his book Zij-i ilkhani (the Ilkhanic Tables). This books contains 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 scientist on this field.

He is also known by Tusi-couple, which resolves linear motion into the sum of two circular motions. He also calculated the value of 51' for the 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 moon crater located on the southern hemisphere is named after him as "Nasireddin".