Hagia Sophia or officially known as Hagia Sophia-i Kebîr Mosque-i Şerîfi, formerly known as Hagia Sophia Church, is a mosque, old basilica, cathedral and museum in Istanbul. It is a basilica planned patriarchal cathedral built by the Byzantine Emperor Justinian I in the old city center of Istanbul between 532-537, and was converted into a mosque by Fatih Sultan Mehmed after the conquest of Istanbul by the Ottomans in 1453. It was converted into a museum with the Decree of the Council of Ministers published in 1934, excavation and renovation works were started and it served as a museum between 1935-2020. In 2020, with the cancellation of the museum status, it gained the status of a mosque again.

Hagia Sophia is a domed basilica type building that combines the central plan in terms of architecture and is considered an important turning point in the history of architecture with its dome passage and carrier system features.

The word "Aya" in the name of Hagia Sophia means "holy, saint". The word "Sofia" comes from the Ancient Greek word sophos, meaning "wisdom".[4] Therefore, the name "Hagia Sophia" means "holy wisdom" or "divine wisdom" and is considered one of the three attributes of God in Orthodoxy.[5] It is stated that approximately 10,000 workers[7][8][9] worked in the construction of Hagia Sophia, which was directed by Isidore of Miletus and Anthemius of Tralles[3][6] and that Justinian I spent a great fortune for this work.[10] A feature of this very old building is that some of the columns, doors and stones used in its construction were brought from earlier structures and temples.[11][12]

In the Byzantine period, Hagia Sophia had a great wealth of "sacred relics". One of these relics is the silver iconostasis, 15 meters high. He witnessed his excommunication by Leo, which marks the beginning of the Schisma in general, the separation of the Eastern and Western churches.

After the church was converted into a mosque in 1453, the mosaics containing human figures were not destroyed (the ones that did not contain them were left as they were), only the mosaics covered with a thin plaster and plastered for centuries were thus spared from natural and artificial destruction. While the mosque was being converted into a museum, some of the plasters were removed and the mosaics were again brought to light. The Hagia Sophia building seen today is also known as the "Third Hagia Sophia" since it is actually the third church built on the same place. The first two churches were destroyed during the riots. The central dome of Hagia Sophia, which was the widest dome of its period, collapsed once (May 7, 558) during the Byzantine period,[14][15] It has never collapsed since Mimar Sinan added buttresses to the building.

Hagia Sophia is a touristic and spiritual center of attraction for people from many faith groups, as well as being both a symbolic and an axis for Christians and Muslims.