Sa'd al-Dawla Tus (Persian: سعدالدوله توس) was the Baduspanid ruler (ustandar) of Rustamdar from 1390 to 1394. He was a son of ustandar Taj al-Dawla Ziyar (r. 1325–1333).[1]
The Baduspanids had been temporarily removed from power after their ruler Adud al-Dawla Qubad (r. 1379–1381) was defeated and killed in 1381 by the Mar'ashis, who incorporated Rustamdar into their own domains, thus extending their sway over all of Mazandaran.[1][2] Rulership over Rustamdar was assigned to the Mar'ashi prince Sayyid Fakhr al-Din.[2]
In 1390, the Mar'ashis installed Tus on the Baduspanid throne to challenge the Afrasiyabid prince Iskandar-i Shaykhi who accompanied the Turco-Mongol ruler Timur (r. 1370–1405), who intended to conquer Mazandaran. However, Tus secretly corresponded with Iskandar-i Shaykhi, and eventually joined the forces of Timur in 1392. The following year (1393), Timur dislodged the Mar'ashis and conquered Mazandaran.[3] He split up their territories between rival families and Timurid governors.[4]
Tus managed to convince Timur to spare the Mar'ashi family; they were deported instead, while Iskandar-i Shaykhi was appointed governor of Mazandaran.[1][3] In 1394, Tus was killed by his nephew Iskandar ibn Gustahm ibn Ziyar. It remains uncertain whether Iskandar succeeded him as ustandar or not. According to the 15th-century historian Zahir al-Din Mar'ashi, Kayumarth succeeded him, but the modern historian Wilferd Madelung doubts this, stating that Kayumarth "must have been too young at this time."[1]