A diocesan administrator (also known as archdiocesan administrator, archiepiscopal administrator and eparchial administrator for the case, respectively, of an archdiocese, archeparchy, and eparchy) is a provisional ordinary of a Catholic particular church.

Under the 1983 Code of Canon Law of the Latin Catholic Church, the college of consultors elects an administrator within eight days after the see is known to be vacant. The college must elect as administrator a priest, bishop, or archbishop at least 35 years old. If the college of consultors fails to elect a priest of the required minimum age within the time allotted, the choice of an administrator passes to the metropolitan archbishop or, if the metropolitan see is vacant, to the senior by appointment of the suffragan bishops of the ecclesiastical province.

If a diocese has a coadjutor bishop, the coadjutor succeeds immediately to the episcopal see upon the previous bishop's death or resignation, and there is no vacancy of the see. The see also does not become vacant if the Pope appoints an apostolic administrator.

Before the election of the 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 administrator has greater powers, essentially those of a bishop or archbishop 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). The administrator remains in charge until a new bishop or archbishop is installed into office ending the sede vacante period or until he presents his resignation to the college of consultors.

Some bishops ruled more than one bishopric for long periods. In any see beside their primary bishopric, they would have to be called an administrator. Nevertheless, in local tradition often they are called bishops in all their bishoprics.

An episcopal conference can transfer the functions of the consultors to the cathedral chapter. In those countries in which the episcopal conference has transferred the functions, the cathedral chapter, and not the consultors, elect the administrator. Capitular election was the default rule before the adoption of the 1983 Code of Canon Law; this old default rule is reflected in the term for the equivalent of an administrator in the 1917 code: vicar capitular.

Since the Investiture Controversy in 11th and 12th centuries the cathedral chapters used to elect the Catholic bishops in the Holy Roman Empire. Prince-bishoprics were elective monarchies of imperial immediacy within the Empire, with the monarch being the respective bishop usually elected by the chapter and confirmed by the Holy See, or exceptionally only appointed by the Holy See. Papally confirmed bishops were then invested by the emperor with the princely regalia, thus the title prince-bishop. However, sometimes the respective incumbent of the see never gained a papal confirmation, but was still invested with the princely power. Also the opposite occurred with a papally confirmed bishop, never invested as prince.

Candidates elected, who lacked canon law prerequisites or papal confirmation, would officially only hold the title diocesan or archdiocesan administrator (but nevertheless colloquially be referred to as prince-bishop). This was the case with Catholic candidates, who were elected for an episcopal see with its revenues as a mere appanage and with all Protestant candidates, who all lacked either the necessary vocational training or the papal confirmation.