Ghiyath al-Dunya wa'l-Din Abu'l-Fath Mas'ud bin Muhammad (c. 1107 – 10 October 1152) was the Seljuq Sultan of Iraq and western Persia in 1133–1152.
Ghiyath ad-Din Masud was the son of sultan Muhammad I Tapar and his wife Nistandar Jahan[1][2][3] also known as Sarjahan Khatun.
In 1133 Masud was able to obtain recognition as sultan from the emirs of Baghdad, and to receive the investiture by caliph al-Mustarshid.
Toghrul, who controlling the eastern provinces of the western Seljuq, launched a military campaign but was defeated by Masud in May 1133.
In 1135 caliph al-Mustarshid contested his authority but, on 14 June of that year, he was defeated and made prisoner at Daimarg, between Hamadan and Baghdad, and killed two months later by the Hashshashins.
As al-Mustarshid's successor, al-Rashid, also rebelled with the support of Zengi, Mas'ud besieged him in Baghdad, forcing him to flee to Mosul, where he was also killed by the Hashshashins.
During his troublesome reign, Masud was forced to accept to delegate his authority to numerous emirs with the iqta', a tax institution which reduced the imperial incomes.