The Patriarchal Cathedral Basilica of Saint Mark (Italian: Basilica Cattedrale Patriarcale di San Marco), commonly known as St Mark's Basilica (Italian: Basilica di San Marco; Venetian: Baxéłega de San Marco), is the cathedral church of the Patriarchate of Venice; it became the episcopal seat of the Patriarch of Venice in 1807, replacing the earlier cathedral of San Pietro di Castello. It is dedicated to and holds the relics of Saint Mark the Evangelist, the patron saint of the city.

The church is located on the eastern end of Saint Mark's Square, the former political and religious centre of the Republic of Venice, and is attached to the Doge's Palace. Prior to the fall of the republic in 1797, it was the chapel of the Doge and was subject to his jurisdiction, with the concurrence of the procurators of Saint Mark for administrative and financial affairs.

The present structure is the third church, begun probably in 1063 to express Venice's growing civic consciousness and pride. Like the two earlier churches, its model was the sixth-century Church of the Holy Apostles in Constantinople, although accommodations were made to adapt the design to the limitations of the physical site and to meet the specific needs of Venetian state ceremonies. Middle-Byzantine, Romanesque, and Islamic influences are also evident, and Gothic elements were later incorporated. To convey the republic's wealth and power, the original brick façades and interior walls were embellished over time with precious stones and rare marbles, primarily in the thirteenth century. Many of the columns, reliefs, and sculptures were spoils stripped from the churches, palaces, and public monuments of Constantinople as a result of the Venetian participation in the Fourth Crusade. Among the plundered artefacts brought back to Venice were the four ancient bronze horses that were placed prominently over the entry.

The interior of the domes, the vaults, and the upper walls were slowly covered with gold-ground mosaics depicting saints, prophets, and biblical scenes. Many of these mosaics were later retouched or remade as artistic tastes changed and damaged mosaics had to be replaced, such that the mosaics represent eight hundred years of artistic styles. Some of them derive from traditional Byzantine representations and are masterworks of Medieval art; others are based on preparatory drawings made by prominent Renaissance artists from Venice and Florence, including Paolo Veronese, Tintoretto, Titian, Paolo Uccello, and Andrea del Castagno.

Several medieval chronicles narrate the translatio, the removal of Saint Mark's body from Alexandria in Egypt by two Venetian merchants and its transfer to Venice in 828/829. The Chronicon Venetum further recounts that the relics of Saint Mark were initially placed in a corner tower of the castrum, the fortified residence of the Doge and seat of government located on the site of the present Doge's Palace. Doge Giustiniano Participazio (in office 827–829) subsequently stipulated in his will that his widow and his younger brother and successor Giovanni (in office 829–832) were to erect a church dedicated to Saint Mark wherein the relics would ultimately be housed. Giustiniano further specified that the new church was to be built between the castrum and the Church of Saint Theodore to the north. Construction of the new church may have actually been underway during Doge Giustiniano's lifetime and was completed by 836 when the relics of Saint Mark were transferred.

Although the Participazio church was long believed to have been a rectangular structure with a single apse, soundings and excavations have demonstrated that St Mark's was from the beginning a cruciform church with at least a central dome, likely in wood. It has not been unequivocally established if each of the four crossarms of the church had a similar dome or were instead covered with gabled wooden roofs.

The prototype was the Church of the Holy Apostles (demolished 1461) in Constantinople. This radical break with the local architectural tradition of a rectangular plan in favour of a centrally planned Byzantine model reflected the growing commercial presence of Venetian merchants in the imperial capital as well as Venice's political ties with Byzantium. More importantly, it underscored that St Mark's was intended not as an ecclesiastical seat but as a state sanctuary.

Remnants of the Participazio church likely survive and are generally believed to include the foundations and lower parts of several of the principal walls, including the western wall between the nave and the narthex. The great entry portal may also date to the early church as well as the western portion of the crypt, under the central dome, which seems to have served as the base for a raised dais upon which the original altar was located.