Ṭūs (Persian: طوس), also spelled as Tous and Toos, is an ancient city in Khorasan, which is located near Mashhad, the current capital city of Razavi Khorasan province, northeast of Iran. Tus was founded in Sassanid era before the emergence of Islam in this country. It was a thriving city before the conquest of Khorasan by Muslims. It consisted of four major cities, Taberan, Noqan, Torqbaz and Radkan.
Harun, the Abbasid caliph, died in Tus and he was buried in Sanabad, a village near this city. On his way back from Marv, Imam al-Rida (a) was martyred and he was buried in Sanabad. Imam (a) also added two verses to a poem composed by Di'bil b. 'Ali al-Khuza'i in which he mentioned he will be buried in Tus.
This city was hugely damaged in the attacks of Mongols and Timurids in which a large number of people were perished. Then most of people moved to live in Sanabad where Imam al-Rida (a) was buried. These events led to marginalization of Tus. However Mashhad became increasingly famous and it became an important Shi'ite city.
The ancient city of Tus is currently located 25 km on the northern part of Mashhad.
Some historians believe the origin of Tus goes back to Achaemenid era and it was called Susia at that time. However, Tus is not mentioned in any historical resources until Sassanid era. It was among the most flourishing cities of Iran in Sassanid era. According to a number of reports, Jamshid Pishdadi founded this city and Tus b. Nowdar rebuilt it later.