| Building Name: | Al-Musta'sim Billah Mosque |
| Religious Affiliation: | Sunni Islam |
| Map Type: | Iraq |
| Coordinates: | 33.3715°N 44.3633°W |
| Location: | Adhamiyah, Baghdad, Iraq |
| Province: | Baghdad Governorate |
| Architecture Style: | Modern, with elements from Abbasid architecture |
| Year Completed: | 2005 (current structure) |
| Capacity: | 200 worshippers |
| Dome Quantity: | 1 (shrine) |
| Minaret Quantity: | 1 |
| Minaret Height: | 30 metres |
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.[1] [2] 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.[3] 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.[4] 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.[5]
The building has an area of 1000 m2 and can accommodate 200 worshippers.[6] Almost no trace of the original mosque remain, having been replaced by a modern structure.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.