The Cathedral Church of Agia Sophia, dedicated to the Holy Wisdom and the Word of God, lies within the old walled core of the historic city, a short distance south of Egnatia Street. In 10^th and 13^th century texts it is referred to as the Great Church, or the Catholic Metropolis (i.e. baptismal cathedral) or the Metropolis. During the Frankish occupation of Thessalonica (1204-1224) the church was temporarily converted into a Latin-rite cathedral, but following the restoration of Byzantine rule it became seat of the city’s Orthodox bishop once more. That until 1523/1524, when it was converted into a mosque. It was restored to Christian worship after the liberation of Thessalonica.
The existing church was built in the 8th century, over the remains of a 5th century five-naved Episcopal basilica destroyed by earthquake in around 620. That had in turn been erected on the site of a 4^th century basilica, most probably with three naves, which was gutted by fire in the second quarter of the 5th century.
The present day building can be classified as a transitional domed cruciform church with peristyle , itself a development of the domed basilica. Combined with the construction date, this peculiarity renders Agia Sophia in Thessalonica one of the most important Byzantine period church buildings.

Its present form, which is the result of successive additions and modifications, remains true to the original bulky yet monumental character, supplemented in the interior by Byzantine mosaics, wall paintings and marble revetments .

All that survives of the sculpted decoration inside the church are the columns and capitals in the north colonnade of the ground floor, which were probably spolia from the earlier basilica on top of which Agia Sophia was built.

The church mosaics date to various periods. The nonfigurative decoration in the sanctuary vault dates to 780-788, as evidenced by the cruciform monograms of Emperor Constantine VI and his mother Irene of Athens, and the inscription mentioning Bishop Theophilus of Thessalonica. Part of the same nonfigurative decoration was the large cross in the sanctuary niche, traces of which can be seen over the halo and beside the shoulders of Mary. The mosaic representation of the Virgin Mary is problematic as regards date. The lower part was previously held to be the oldest (9th century), with the upper one dating to a later phase, in the 11th or 12th century. More recent studies have shown that the main body does not differ from the lower section, so it has been argued that the scene is contemporary with that in the dome.

The large monumental representation of the Ascension in the centre of the dome dates to the end of the 9th century, most probably to 885, if we accept that the Archbishop Paul mentioned in an inscription in the decorative band surrounding the subject is to be identified as the like-named Metropolitan of Thessalonica, who had ties with Ecumenical Patriarch Photios. The scene depicts Christ at the centre of a circular mandorla supported by flying angels; lower down at the base of the dome is a rendition of olive trees in a rocky landscape, where the figures of the apostles are walking. The Virgin Mary can be seen among them, flanked by two archangels pointing towards the Divine Presence in the heavens.
The frescoes in the arcs of the west narthex wall date to the 11th century. Figures of monks are still visible, together with local saints from Thessalonica such as Agia Theodora.

The painted decoration imitating marble revetments owes its existence to repairs carried out during the Ottoman period. The building was damaged by fire in 1890, and restored by Charles Diehl in 1907-1909. Yet it was the impact of the 1978 earthquakes that led to systematic study and maintenance, both vital steps in bringing to the fore what is a very special monument, a part of World Cultural Heritage and an important place of worship in present-day Thessalonica.