The Church of San Vitale, located in northwestern Ravenna, is a remarkable example of Late Antique architecture. While it was begun when Ravenna was still under Ostrogothic rule, it is Byzantine in style and similar to the Church of Saints Sergius and Bacchus built by Justinian in Constantinople around the same time. It first served as the martyrium of Saint Vitalis, Ravenna's patron saint, and was later part a Benedictine monastery some time before the mid-tenth century until it was dissolved in 1860. It is now one of the eight Early Christian monuments of Ravenna, which were added to UNESCO World Heritage List in 1996.

While it was built during the reign of Justinian (527-565), there are many aspects about San Vitale that remain uncertain, including the precise date of its construction and its connection to Constantinople. Work on the church began under Bishop Ecclesius (521-532) after returning from Constantinople and was financed by a Greek speaker from the east named Julianus Argentarius. Even though Ravenna was then the capital of the Ostrogoth kingdom, Byzantine political influence was growing there during the regency of Queen Amalasuntha (526-534) and would ultimate be conquered in 540. The church, though, was consecrated much later by Bishop Maximian in 547. Bishop Victor (538-545) probably played a substantial role in the construction of San Vitale, as his monograms can be found on the impost blocks of its capitals. This lengthy period of construction, possibly due to the Gothic War and the Justinianic plague, has led to debate, particularly as it relates to the church of Sts. Sergius and Bacchus in Constantinople, another double shell building with a very similar design. As Bishop Ecclesius left Constantinople before Sergius and Bacchus was completely, it suggests that it could not have served as a direct model. There were, in fact, other churches with somewhat similar plans (including San Lorenzo in Milan and even San Costanza outside Rome) that also could have influenced the design of both churches.

It is clear that the design and decoration of San Vitale was directly influenced by the latest trends in Constantinople. It is extensively decorated with marble from Proconnesius, an island in the vicinity of Constantinople, and reflects Constantinopolitan designs also found in Sergius and Bacchus. San Vitale also has split-palmette capitals that first were used in the Church of St. Polyeuktos in Constantinople built a short time earlier. San Vitale was built as the martyrium of Saint Vitalis, not as a palatial chapel, as has occasionally been suggested. Nonetheless, Charlemagne used San Vitale for the model of the chapel for his palace in Aachen in the 8th century, and the Chrysotriklinos (“Golden Hall”) of the Great Palace, build during the reign of Justin II (565–578), also followed a similar plan. Once it was completed, San Vitale was part of a remarkable complex of buildings that included the Santa Croce complex and Santa Maria Maggiore. San Apollinare in Classe, which was begun by Bishop Ursicinus (534-536) and consecrated by Bishop Maximian in 549, is also a contemporary church.

San Vitale, unlike other churches in Ravenna, is truly Byzantine in design. This octagonal double-shelled building is strikingly similar to the Church of Sts. Sergius and Bacchus in Constantinople that was built around the same time.  It has a diameter around 40 meters, while its octagonal naos has eight inner piers joined by arches that support a dome on squinches. Two-story, triple-arched semicircular exedrae between the piers extend outward and are surrounded on two levels by an ambulatory and a second-story gallery.  The columns at the gallery level are slightly shorter than those of the ground level which serves to accentuate the sense of verticality. On its eighth side is an arch leading to an elaborately decorated high vaulted presbytery (bema) and apse with a circular interior and polygonal exterior. The apse is flanked by pairs of small apsed rooms and round domed chapels. The chapels, which have rectangular arcosolia to the east and niches on each side, were used as mausolea as well as chapels with altars and screens. The chapel to the south of the apse served as the mausoleum of bishops Ecclesius, Ursicinus, and Victor.

Its similarity to Sergius and Bacchus has also led questions about the origin of its architect. While similar in plan and design, there are some notable differences in San Vitale. All niches of San Vitale are semicircular, while the niches of Sts. Sergius and Bacchus are of alternating rectangular and semicircular plan, with arcades only at the gallery level.  The proportions of San Vitale are also much steeper than Sergius and Bacchus. Most notably its dome is made of tubi fittili, hollow terracotta tubes inserted into one another, rather than the brickwork typical of Constantinople. Of course many elements of San Vitale were imported from the area of Constantinople, including marble columns, capitals, and paneling. Its brickwork is also similar to Byzantine masonry, with thin bricks and wide mortar joints rather than the short, thick bricks prevalent at the time in northern Italy. Even if its architect was sent from the East by Constantinople, certainly local craftsmen at the very least were essential for its construction. Conversely, even if San Vitale was built by a local architect, it is clear that many features of its design were influenced by or even finalized at Constantinople.

San Vitale was designed with doors on all seven walls of the octagon, including two leading to the narthex. The apse has three broad windows at ground level and another three windows above the springing of the apse, while the six walls of the ambulatory and galleries have two or three windows. Its dome (9 m diameter, 28.7 m height) is pierced by eight large windows that provide direct light to the naos. The light from all of these sources creates the effects of light and shadow throughout the building, while the multiple doors and window also give the space a sense of openness. It has a long, double-apsed narthex to the southwest, which once connected to a colonnaded atrium. There are also triangular vestibules between the narthex and octagon that connect to round stair towers leading to the gallery. Both the narthex and the atrium are asymmetrical aligned, both to each other and the apse, possibly due to a preexisting structure or street that prevented construction on an axis. As the exterior corners of the octagon were heavily buttressed to provide support for the dome, the off-axis narthex also helped provide similar support.

Most the original building has survived except for the medieval vaults that replaced its wooden ceilings over the ambulatory and gallery. Major changes were made in the vaulting of the ambulatory and gallery, while its exterior buttresses were added perhaps in the late 12th century. Its southern stair tower was also transformed into a campanile, which collapsed in 1688. Major restoration occurred in the 16th century, while several chapels were added to the church in the 18th century. In the mid 19th century, its mosaics were restored. In the early 20th century, excavations were made while the church was reconstructed in its presumed original form. Additional restorations at later dates included lowering the pavements to its original level and reconstituting the narthex.  A small rectangular chapel was also discovered under the original floor of the church, which is believed to the original chapel dedicated to St. Vitalis in the 5th century.

San Vitale was lavishly decorated with marble wall revetment, mosaic and marble pavements, columns and impost capitals, stucco decoration under the arches, painted glass windows, and figural mosaics in the presbytery (bema) and apse. The Proconnesian marble elements, including columns, capitals, and chancel furnishings, followed the latest Constantinopolitan styles. There are fourteen Proconnesian columns for the triple arcades at the ground level, fourteen slightly smaller columns on the level of the gallery, eight columns for the triple arcades leading to the presbytery, and two colonnades for the windows above the apse. Most imposts are carved on their main sides, with monograms of Bishop Victor on the ground floor arcade, and two imposts with the monogram of Julian the banker on the gallery level. The imposts of the presbytery have pairs of lambs flanking a cross facing the altar and doves flanking vases on the opposite sides. There are twenty paneled impost capitals and sixteen mask acanthus capitals, along with fold capitals (as in Sergius and Bacchus) on the north side of the presbytery gallery. The capitals on the ground level have basket patterns with split-palmette (as in St. Polyeuktos), while the capitals in the presbytery have acanthus borders surrounding scrolls of a different type of acanthus as well as some geometric motifs. All of the capitals and impost blocks were originally painted, as can be seen in the restored example in the presbytery. San Vitale also had Proconnesian marble liturgical furnishings, including an altar, ciborium, and transennae (now in the Museo Nazionale). The frontal panel on the altar today, decorated with a large cross flanked by sheep and hanging crowns, was probably the back of the original altar.

The original columns, capitals, and impost blocks are still in place, but most of the wall and floor coverings in the church today are modern reconstructions. Elaborate cornices run around the walls and piers of most of the building. Marble revetment, which originally covered the lower sections of the wall, was made of Proconnesian marble (white with gray veins) and marble from lasos in Caria (deep red with white veins). High quality stucco was also used to decorate some sections of the church, as traces can be found in some of the arcades, window arches, and in the southern triangular vestibule of the narthex. The wall below the mosaics in the apse was originally decorated with opus sectile, with large roundels of porphyry, with smaller marble pieces and mother of pearl. The current decoration below the apse and the synthronon of the apse are reconstructions. The floors were decorated with mosaic in eight triangular slices radiating out from a central medallion. Six segments were remade in the 1540s, while the other two were remade in the early 18th century. In 1931, excavation revealed two segments of the original floor, after which the entire level was lowered to the original level, with the two 18th century segments being replaced with restored original mosaic. The apse and presbytery floor was created between the 1910s-1930s. 

The apse, the walls of the presbytery and the triumphal arch are covered with mosaics. The mosaics, which have been restored many times, probably date a couple of years before the church was consecrated by Bishop Maximian in 547, though the panel with Justinian and Bishop Maximian was perhaps made around the time of consecration. It is highly likely that the dome originally had mosaics as well, though no traces have survived. At least two styles of mosaics have been identified; the first style uses gold for the backgrounds and glass tesserae for human figures, while the second style uses green for the backgrounds and uses stone for the skin of human figures. Much of the imagery is related to the celebration of the Eucharist that took place in this space, including the magnificent depictions of Justinian and Theodora with their court.