The Al-Musta'sim Billah Mosque (Arabic: مسجد المستعصم بالله), formerly the Abu Rabi'ah Mosque, is an Islamic mosque and shrine located in the district of al-A'dhamiyya in Baghdad, Iraq. A historic structure renovated in modern times, it contains the tomb of the last Abbasid Caliph, al-Musta'sim. It was also known as Mashhad al-Wu'ud, before it became a mosque.

Originally, the site was the location of a shrine known as Mashhad al-Wu'ud, built over the grave of Ahmad al-Sabti, who was the son of the Abbasid Caliph Harun al-Rashid and a Sufi mystic. The Abbasid Caliph, al-Musta'sim was killed by Hulegu Khan's forces in the 1258 Siege of Baghdad, his remains were transferred to this location and reburied. His daughter (or granddaughter) Rabi'ah was also buried here later on, and a shrine was built for her too, hence the name of the mosque was the Abu Rabi'ah Mosque.

Before discovering the Caliph's tomb, it was traditionally believed that al-Musta'sim was buried in the Mausoleum of Umar Suhrawardi. In 1993, an archaeological excavation was conducted at the mosque. The historian Imad Abd al-Salam Ra'uf, who was involved in the excavations, was able to find the tombstone of the deceased Caliph. The mosque and the attached tomb were renovated in 2005. It was almost completely rebuilt with modern architecture. A room for the Imam and other necessities were added to the mosque's structure.

The building has an area of 1000 m2 and can accommodate 200 worshippers. Almost no trace of the original mosque remain, having been replaced by a modern structure. A small dome remains over the tomb of Caliph al-Musta'sim, and all other historical features of the tomb have disappeared.

• List of mosques in Iraq
• List of mosques in Baghdad