An apostolic nuncio (Latin: nuntius apostolicus; also known as a papal nuncio or simply as a nuncio) is an ecclesiastical diplomat, serving as an envoy or a permanent diplomatic representative of the Holy See to a state or to an international organization. A nuncio is appointed by and represents the Holy See, and is the head of the diplomatic mission, called an apostolic nunciature, which is the equivalent of an embassy. The Holy See is legally distinct from the Vatican City or the Catholic Church. In modern times, a nuncio is usually an archbishop.

An apostolic nuncio is generally equivalent in rank to that of ambassador extraordinary and plenipotentiary, although in Catholic countries the nuncio often ranks above ambassadors in diplomatic protocol. A nuncio performs the same functions as an ambassador and has the same diplomatic privileges. Under the 1961 Vienna Convention on Diplomatic Relations, to which the Holy See is a party, a nuncio is an ambassador like those from any other country. The Vienna Convention allows the host state to grant seniority of precedence to the nuncio over others of ambassadorial rank accredited to the same country, and may grant the deanship of that country's diplomatic corps to the nuncio regardless of seniority. The representative of the Holy See in some situations is called a Delegate or, in the case of the United Nations, Permanent Observer. In the Holy See hierarchy, these usually rank equally to a nuncio, but they do not have formal diplomatic status, though in some countries they have some diplomatic privileges.

In addition, the nuncio serves as the liaison between the Holy See and the Church in that particular nation, supervising the diocesan episcopate (usually a national or multinational conference of bishops which has its own chairman, elected by its members). The nuncio has an important role in the selection of bishops.