Abu Mansur al-Faḍl ibn Ahmad al-Mustazhir (Arabic: أبو منصور الفضل بن أحمد المستظهر; 1092 – 29 August 1135) better known by his regnal name Al-Mustarshid Billah (Arabic: المسترشد بالله) was the Abbasid caliph in Baghdad from 1118 to 1135.

His full name was Al-Fadl ibn Ahmad al-Mustazhir and his Kunya was Abu Mansur.

He achieved more independence as a ruler while the Seljuk sultan Mahmud II was engaged in war in the East.

In 1122, al-Mustarshid deposed and imprisoned his vizier Amid al-dawla Jalal al-Din Hasan ibn Ali.

[3] Ahmad then retired to a school in Baghdad which was founded by his father, the Nezamiyeh, where he lived the last 25 years of his life, dying in 1149/1150.

[4] In 1123, Banu Mazyad chieftain Dubais ibn Sadaqah tried to take advantage of the momentary lack of power and, after plundering Bosra, attacked Baghdad together with a young brother of the sultan, Ghiyath ad-Din Mas’ud (known as Mas'ud).

[citation needed] After the death of Mahmud II, a civil war broke out in the Seljuk western territories.

[citation needed] Zengi now resumed operations in Syria and, in 1134, laid siege to Damascus, but was induced, partly by the bravery of the enemy, partly at the instance of the caliph, to whom Zengi had made some concession in the public prayers, to relinquish the attempt.

[citation needed] Not long after the siege of Damascus, al-Mustarshid launched a military campaign against Seljuk sultan Mas'ud, who had obtained the title in Baghdad in January 1133 by the caliph himself.