Vatican City, officially the Vatican City State (Italian: Stato della Città del Vaticano; Latin: Status Civitatis Vaticanae), often shortened as the Vatican, is a landlocked sovereign city-state. Ruled by the pope, it is an enclave within Rome and serves as the administrative centre of the Catholic Church. Vatican City is governed by the See of Rome, commonly known as the Holy See, itself a sovereign entity under international law, which maintains its temporal power, governance, diplomacy, and spiritual independence. Vatican is also used as a metonym for the Holy See, which is the central governing body of the Catholic Church and Vatican City, comprising the pope and the Roman Curia. The independent state of Vatican City came into existence in 1929 via the Lateran Treaty between the Holy See and the Kingdom of Italy, which spoke of it as a new creation, not as a vestige of the much larger Papal States (756–1870), which had previously encompassed much of Central Italy.

With an area of 49 hectares (121 acres) and a population of about 882 in 2024, it is the smallest sovereign state in the world both by area and by population. It is among the least populated capitals in the world. As governed by the Holy See, Vatican City State is an ecclesiastical or sacerdotal-monarchical state ruled by the pope, who is the bishop of Rome and head of the Catholic Church; the highest state functionaries are all Catholic clergy of various origins. The Holy See dates to early Christianity and is the principal episcopal see of the Catholic Church, which in 2018 had about 1.329 billion baptized Catholics in the world, in the Latin Church and 23 Eastern Catholic Churches. After the Avignon Papacy (1309–1377) the popes have mainly resided at the Apostolic Palace within what is now Vatican City, although at times residing instead in the Quirinal Palace in Rome or elsewhere.

Vatican City contains religious and cultural sites such as St Peter's Basilica, the Sistine Chapel, the Vatican Apostolic Library, and the Vatican Museums. They feature some of the world's most famous paintings and sculptures. The economy of Vatican City is supported financially by donations from Catholic believers, by the sale of postage stamps and souvenirs, fees for admission to museums, and sales of publications. Vatican City has no taxes, and items are duty-free.

The name Vatican City was first used in the Lateran Treaty, signed on 11 February 1929, which established the modern city-state named after Vatican Hill, the geographic location of the state within the city of Rome. "Vatican" itself is derived from the name of an Etruscan settlement, Vatica or Vaticum, located in the general area the Romans called Ager Vaticanus, "Vatican territory".

The Italian-language name of the city is Città del Vaticano or, more formally, Stato della Città del Vaticano, meaning 'State of Vatican City'. Its Latin name is Status Civitatis Vaticanae; this is used in official documents by the Holy See, the Church and the pope.

The name "Vatican" was already in use in the time of the Roman Republic for the Ager Vaticanus, a marshy area on the west bank of the Tiber across from the city of Rome, located between the Janiculum, the Vatican Hill and Monte Mario, down to the Aventine Hill and up to the confluence of the Cremera creek. The toponym Ager Vaticanus is attested until the 1st century AD: afterwards, another toponym appeared, Vaticanus, denoting an area much more restricted: the Vatican Hill, today's St. Peter's Square, and possibly today's Via della Conciliazione. Because of its vicinity to Rome's archenemy, the Etruscan city of Veii (another naming for the Ager Vaticanus was Ripa Veientana or Ripa Etrusca), and for being subjected to the floods of the Tiber, the Romans considered this originally uninhabited part of Rome dismal and ominous.

The particularly low quality of Vatican wine, even after the reclamation of the area, was commented on by the poet Martial (AD 40 – c. AD 102). Tacitus wrote that in AD 69, the Year of the Four Emperors, when the northern army that brought Vitellius to power arrived in Rome, "a large proportion camped in the unhealthy districts of the Vatican, which resulted in many deaths among the common soldiery; and the Tiber being close by, the inability of the Gauls and Germans to bear the heat and the consequent greed with which they drank from the stream weakened their bodies, which were already an easy prey to disease".

During the Roman Empire, many villas were constructed there, after Agrippina the Elder (14 BC – 18 October AD 33) drained the area and laid out her gardens in the early-1st century AD. In AD 40 her son, the emperor Caligula, built in her gardens a circus for charioteers. It was later completed by the emperor Nero and named the Circus Gaii et Neronis, usually called, simply, the Circus of Nero.