The Papal Basilica of St. Peter, commonly known as St. Peter's Basilica, is a Catholic church located in the Vatican City.

The basilica has the largest interior space of a Christian church in the world, it is 193 m long, 44.5 m high, and covers an area of 2.3 hectares. The height conferred by its dome makes its figure dominate the skyline of Rome. It is considered one of the most sacred places in Catholicism. It has been described as "occupying a unique position in the Christian world", and as "the largest of all the churches of Christianity". It is one of the four major basilicas and one of the churches that must be visited on the pilgrimage of the seven churches of Rome to attain plenary indulgence.

In Catholic tradition, the basilica is located on the burial place of St. Peter, who was one of the twelve apostles of Jesus of Nazareth, the first bishop of Antioch, the first bishop of Rome and, therefore, the first of the pontiffs. Tradition and historical and scientific evidence support that the tomb of the Saint is directly under the high altar of the basilica. Also most of the popes have been buried in St. Peter's since early Christian times.

Churches have existed on the site of the basilica since the 4th century. The construction of the present building, on the site of the ancient Constantinian basilica, began on April 18, 1506, by order of Pope Julius II, and was completed on November 18, 1626. Some of the most renowned architects in history worked on it, such as Donato Bramante, Michelangelo and Bernini.

Despite popular belief, St. Peter's is not a cathedral, since the Archbasilica of St. John Lateran is the true cathedral of Rome.

St. Peter's Basilica is one of the four papal basilicas or major basilicas of Rome, the others being the Archbasilica of St. John Lateran, the Basilica of St. Mary Major and the Basilica of St. Paul Outside the Walls.

St. Peter's Basilica is neither the official seat of the pope, nor is it the first basilica of Rome, as this honor is held by the Archbasilica of St. John Lateran; nor is it the parish of Vatican City, as it is currently the Pauline Chapel that functions as such. However, it is the main pontifical church where most of the papal ceremonies are held due to its size, its proximity to the papal residence and its location within the Vatican City.

The Chair of St. Peter is an ancient chair that is supposed to have been used by St. Peter, but was actually a gift from Charles the Bald, and used by several popes. It occupies an elevated position in the apse, in a bronze reliquary supported by figures of the Doctors of the Church, and illuminated by a stained glass window depicting the Holy Spirit.

After the crucifixion and resurrection of Jesus, in the second quarter of the first century, it is recorded in the biblical book of the Acts of the Apostles that one of his twelve disciples, Simon Peter, a fisherman from Galilee, occupies a position of leadership among his followers, having great importance in the foundation of the Christian Church.

Simon Peter, after a ministry of about thirty years, traveled to Rome. In the year 64, during the reign of Emperor Nero, the Christians were held responsible for the great fire of Rome, for which Peter was martyred, along with many other Christians. He was crucified upside down, at his own request, because he considered himself unworthy to die in the same way as Christ did. The crucifixion took place near the Egyptian obelisk at one end of Nero's Circus. This obelisk was brought from the city of Heliopolis by Caligula in 37 A.D. It now stands in St. Peter's Square, and is venerated as a "witness" to the death of the apostle. It is one of several ancient obelisks in Rome.

According to tradition, Peter's remains were buried outside the Circus, on the Vatican Hill, advancing along the Via Cornelia, which started from the Circus, less than 150 m from the place of his death. Peter's tomb was marked by a red rock, symbol of his name, which was used by Christians to identify it and, at the same time, was meaningless to non-Christians, thus avoiding possible reprisals. Years later, a sanctuary or "cella memoriae" was built on this site, which almost 300 years later became the ancient Basilica of St. Peter.

In 1939, during the papacy of Pius XII, a 10-year archaeological investigation was carried out in the crypt of the basilica, which had remained inaccessible since the 9th century. In fact, the area covered by Vatican City had been a cemetery since before the construction of Nero's Circus. In addition, the victims of the circus executions were buried there, as well as numerous Christians who chose to be buried near the apostle. The excavations revealed, at different levels, the remains of the sanctuaries that existed at the time of Clement VIII (1594), Callixtus II (1123) and Gregory I (590-604). All the constructions were on an edicule containing fragments of bones in a cloth dyed purple with gold ornaments. Although it could not be determined with certainty that the bones were those of Peter, the clothing suggested a burial of great importance. On December 23, 1950, Pope Pius XII announced the discovery of the tomb of St. Peter the Apostle.

The ancient basilica was built on the site of Peter's tomb, not far from Nero's circus. The works were ordered by Emperor Constantine, at the expense of Pope Sylvester I; they began between 326 and 330, and were completed 30 years later. Many emperors were crowned before the high altar of this church, such as Charlemagne, on whom Pope Leo III imposed the imperial crown on Christmas Day 800.

At the end of the 15th century, after the period of the Avignon papacy, the early Christian basilica was quite deteriorated and threatened to collapse. The first pope to consider rebuilding, or at least making radical changes, was Nicholas V in 1452. He commissioned work on the old building to Leon Battista Alberti and Bernardo Rossellino, who was charged with designing the most important changes. In his project, Rossellino kept the longitudinal body of five naves covered with vaulted ceilings and renovated the transept with the construction of a wider apse to which he added a choir; this new intersection between the transept and the apse would be covered with a vault. This configuration devised by Rossellino influenced Bramante's later project. The works were interrupted three years later, at the pope's death, when the walls were only one meter above the ground. However, the pope ordered the demolition of the Colosseum in Rome and, at the time of his death, 2,522 carloads of stone had been transported for use in the new building.