Sede vacante is an expression, used in the Canon Law of the Roman Catholic Church, that refers to the vacancy of the episcopal see of a particular church. It is Latin for "the seat being vacant" (the ablative absolute to sedes vacans "vacant seat"), that is, the cathedra of the particular church.

For dioceses other than the Diocese of Rome

This means that for a diocese the diocesan bishop has either died, resigned, transferred to a different diocese, or lost his office and a replacement has not yet been named. If there is a coadjutor bishop for the church, then this period does not take place, as the coadjutor bishop (or coadjutor archbishop, in the case of an archdiocese) immediately succeeds to the episcopal see.

It is not to be confused with Sedevacantism, an extreme strand of the Catholic traditionalist movement.

Within eight days after the see is known to be vacant, the college of consultors (or the cathedral chapter in some countries) is obliged to elect a diocesan administrator. The administrator they choose must be a priest or bishop who is at least 35 years old.

If the college of consultors fails to elect a qualifying person within the time allotted, the choice of diocesan administrator passes to the metropolitan archbishop or, if the metropolitan see is vacant, to the seniormost by appointment of the suffragan bishops.

Before the election of the diocesan administrator of a vacant see, the governance of the see is entrusted, with the powers of a vicar general, to the auxiliary bishop, if there is one, or to the senior among them, if there are several, otherwise to the college of consultors as a whole. The diocesan administrator has greater powers, essentially those of a bishop except for matters excepted by the nature of the matter or expressly by law. Canon law subjects his activity to various legal restrictions and to special supervision by the college of consultors (as for example canons 272 and 485).
Vicars general and episcopal vicars lose their powers sede vacante if they are not bishops; the vicars that are themselves bishops retain the powers they had before the see fell vacant, which they are to exercise under the authority of the administrator.

Vacancy of the Holy See

The expression sede vacante may refer as well to the vacancy of the Holy See, which occurs after the death or resignation of a pope. In this case the particular church is the Diocese of Rome and the "vacant seat" is the cathedra of Saint John Lateran, the cathedral church of the bishop of Rome. During this period, the Holy See is administered by a regency of the College of Cardinals.

According to Universi Dominici Gregis, the government of the Holy See sede vacante (and therefore of the Catholic Church) falls to the College of Cardinals, but in a very limited capacity. At the same time, all of the heads of the Roman Curia resign their offices. The exceptions are the Cardinal Camerlengo, who is charged with managing the property of the Holy See, and the Major Penitentiary, who continues to exercise his normal role. If either has to do something which normally requires the assent of the Pope, he has to submit it to the College of Cardinals. Papal legates continue to exercise their diplomatic roles overseas, and the Vicar General of Rome continues to exercise his pastoral role over the diocese of Rome during this period. The postal administration of the Vatican City State prepares and issues special postage stamps for use during this particular period, known as "sede vacante stamps".

The coat of arms of the Holy See also changes during this period. Instead of the papal tiara over the keys, the tiara is replaced with the umbraculum or ombrellino in Italian. This symbolizes both the lack of a Pope and also the governance of the Camerlengo over the temporalities of the Holy See. As further indication, the Camerlengo ornaments his arms with this symbol during this period, which he subsequently removes once a pope is elected. The arms of the Camerlengo appear on commemorative euro coins minted during this period, which are legal tender in all Eurozone member states.

The interregnum is usually highlighted by the funeral mass of the deceased pope, the general congregations of the college of cardinals for determining the particulars of the election, and finally culminated in the conclave to elect a successor. Once a new pope has been elected (and ordained bishop if necessary) the sedes is no longer vacant, so this period then officially ends. Afterward occurs the Papal Installation or Papal Coronation, depending on the form of inauguration and investiture a new pope chooses, and the formal possession of the cathedra of the Basilica di San Giovanni in Laterano.
Cardinals present in Rome are required to wait at least fifteen days after the start of the vacancy for the rest of the college before they can hold the conclave to elect the new Pope. However, after twenty days have elapsed, they must hold the conclave even if cardinals are missing. Historically, sede vacante periods have often been quite lengthy, lasting many months due to lengthy deadlocked conclaves. For many years through 1922 the period from the death of the Pope to the start of the conclave was shorter, but after William Henry Cardinal O'Connell had arrived just too late for two conclaves in a row, Pope Pius XI extended the time limit. With the very next conclave in 1939, cardinals began to travel by air.

The most recent period of sede vacante of the Holy See began at 19:37 UTC, April 2, 2005, due to the death of Pope John Paul II, and concluded with the election of Pope Benedict XVI at 16:05 UTC, April 19, 2005.

List of sede vacante periods in the Holy See since the 19th century