In the north of the city is the Church of Hosios David, formerly the katholikon of a monastery. It is one of the 15 Paleochristian and Byzantine Monuments of Thessaloniki that were included in the UNESCO World Heritage List in 1988.
The dedication of this church to Hosios David is completely arbitrary, and was made as recently as 1921, when the building was reconsecrated as a Christian church, having been converted into a mosque in the Turkish period. It is known, however, from a late 9th century text that at that period the katholikon was dedicated to Christ the Savior of Latomos, after the miraculous discovery of the mosaic in the sanctuary apse. 
The church, which was probably dedicated to the prophet Zacharias, was founded in the late 5th century. According to tradition, the church was founded by Theodora, the daughter of the emperor Maximianus. She supposedly founded the church as a bathhouse and concealed its mosaic to dispel her mother's suspicions of her conversion to Christianity. The mosaic was probably concealed during the period of the iconoclastic controversy. The mosaic was said to have been discovered in 9th century, during the course of an earthquake, when the oxhide fell and revealed the mosaic before the eyes of a monk. The katholikon was renewed in the 12th century and decorated with new wall-paintings. The painted decoration of the katholikon seems to have been completed in the late 13th - early 14th century. Exactly when it was converted into a mosque is not known, though it is conjectured that this happened during the 16th century, when many other churches in Thessaloniki were also converted into mosques.
In its original form, the church has a square ground plan with a semicircular apse at the east. Inside the square was inscribed a Greek cross, and four corner bays were created in the spaces between the arms of the cross and the outer walls. The arms of the cross were vaulted, the central bay was roofed with a low dome, without windows, and the corner bays with a kind of cross-vault, The interior was amply lit by two-light windows set in the ends of the arms of the cross, and a two-light window in the sanctuary apse. The west section of the original building is missing today, apparently having been destroyed at the time of the conversion of the church into a mosque, since the base of the minaret has been found in the north-west bay. The present humble church preserves one of the most important mosaic and painted decorative ensembles not only in Thessaloniki, but of Early Christian and Byzantine art in general. 
The semidome over the sanctuary apse has a scene of a Theophany. The centre of the composition is occupied by the youthful figure of Christ seated on an arch symbolizing heaven, within a large, brilliant glory. The four rivers of Paradise (the Phison, Geon, Tigris and Euphrates) flow beneath his feet, their waters feeding the river Chobar (or Jordan), which spreads over the bottom of the com-position. In the river swim multi-colored fish; the aged figure depicted is undoubtedly the personification of the river, based on models found in classical art. The glory is surrounded by an angel, an eagle, a lion and a calf, holding closed books and symbolizing the evangelists Matthew, John, Mark and Luke respectively. The Theophany is witnessed by two figures that have been identified with prophets. The figure standing at the left, in an attitude of fear and ecstasy, is, according to the predominant view, the prophet Ezekiel, and the seated figure at the right, in an attitude of reflection, is Habakkuk. The triumphant appearance of Christ, and the inscription on his scroll determine the meaning of the scene, which is a reference to the Salvation of mankind and the Last Judgment. The mosaic is dated to the last quarter of the 5th century, according to the view most widely accepted by scholars. The rich, brilliant color scale, the combination of spirituality and realism, particularly in the treatment of the animals, the naturalistic rendering of the landscape, which is very close to the Greco-Roman concept, all make this one of the most brilliant works that have survived to us from the art of its period. 
The wall-paintings discovered relatively recently on the south vault, which have been dated to the third quarter of the 12th century, are of equal artistic importance. Traces of the Presentation of Christ in the Temple and the Transfiguration have been preserved, with the scenes of the Nativity and the Baptism intact above them. In the multi figural scene of the Nativity the main axis of the depiction is defined by the reclining figure of the Virgin, while the figure of Joseph, seated on a saddle, lost in contemplation with an expression of deep melancholy, has an impressive maturity. In the episode of the Bathing of Christ, the figure of the mature mid-wife, who is bathing the Infant with her sleeves rolled up, is counterbalanced by the graceful figure of Salome in an affected, lively pose. In the Baptism, the axis of the scene is defined by the naked figure of Christ, whose volumes and posture recall those of ancient statues. The ascetic figure of John the Baptist at the left, and the majestic pose and dress of the angel at the right create a counter-point full of vigor and meaning. Both scenes convey a rare sense of landscape and atmosphere and, together with the new concept of the Classical tradition to which they give expression, form a unique painted ensemble, one of the finest creations of the art of the Komnenian period. Of the painted decoration of the church in the late 13th - early 14th century, a few fragmentary scenes are preserved on the east wall of the north vault: the Entry into Jerusalem, the Prayer on the Mount of Olives, and the Virgin of the Passion.