The mitre (Commonwealth English) or miter (American English; see spelling differences; both pronounced /ˈmaɪtər/ MY-tər; Greek: μίτρα, romanized: mítra, lit. 'headband') is a type of headgear now known as the traditional, ceremonial headdress of bishops and certain abbots in traditional Christianity. Mitres are worn in the Catholic Church, Malankara Orthodox Syrian Church (IOC), Malankara Syrian Orthodox Church (Jacobites), Eastern Orthodox Church, Oriental Orthodox Churches, the Anglican Communion, some Lutheran churches, for important ceremonies, by the Metropolitan of the Malankara Mar Thoma Syrian Church, and also, in the Catholic Church, all cardinals, whether or not bishops, and some Eastern Orthodox archpriests.

Μίτρα, mítra (Ionic μίτρη, mítrē) is Greek, and means a piece of armour, usually a metal guard worn around the waist and under a cuirass, as mentioned in Homer's Iliad. In later poems, it was used to refer to a headband used by women for their hair, and a sort of formal Babylonian headdress, as mentioned by Herodotus (Histories 1.195 and 7.90). It also refers to a kind of hairband, such as "the victor's chaplet at the games", a headband and a badge of rank at the Ptolemaic court, an oriental headdress, perhaps a kind of turban, worn "as a mark of effeminacy", a diadem, the headdress of the priest of Heracles, or the headdress of the High Priest of Israel referenced in the Septuagint (Greek) text of Exodus 29:6.

In its modern form in Western Christianity, the mitre is a tall folding cap, consisting of two similar parts (the front and back) rising to a peak and sewn together at the sides. Two short lappets always hang down from the back.

The camelaucum (Greek: καμιλαύκιον, kamilaukion), the headdress that both the mitre and the papal tiara stem from, was originally a cap used by officials of the Imperial Byzantine court. "The tiara [from which the mitre originates] probably developed from the Phrygian cap, or frigium, a conical cap worn in the Graeco-Roman world. In the 10th century the tiara was pictured on papal coins." Other sources say the tiara developed the other way around, from the mitre. In the late Empire it developed into the closed type of Imperial crown used by Byzantine Emperors (see illustration of Michael III, 842–867).

Worn by a bishop, the mitre is depicted for the first time in two miniatures of the beginning of the eleventh century. The first written mention of it is found in a Bull of Pope Leo IX in the year 1049. By 1150 the use had spread to bishops throughout the West.^{[citation needed]}

In the Church of England, the mitre fell out of use after the Reformation, but was restored in the late 19th and early 20th centuries as a result of the Oxford Movement, and is now worn by most bishops of the Anglican Communion on at least some occasions. In The Episcopal Church of the United States, the first Presiding Bishop, Samuel Seabury, wore a mitre as early as 1786. The mitre is also worn by bishops in a number of Lutheran churches, for example the Evangelical Lutheran Church of Latvia, the Church of Sweden, and the Lutheran Church in Great Britain.

In the Catholic Church, ecclesial law gives the right to use the mitre and other pontifical insignia (crosier, pectoral cross, and ring) to bishops, abbots, cardinals, and those canonically equivalent to diocesan bishops who do not receive episcopal ordination, such as married Ordinaries of the Anglican ordinariates, who are not able to be raised to the episcopacy. The principal celebrant presents the mitre and other pontifical insignia to a newly ordained bishop during the Rite of Ordination of a Bishop and to a new abbot during the Rite of Blessing of an Abbot. In the case of a person who is canonically equivalent to a diocesan bishop but does not receive episcopal ordination, this presentation normally occurs during a public installation as the ordinary of his jurisdiction. Catholic ecclesial law also permits former Anglican bishops received into full communion and subsequently ordained to the order of presbyter in the Catholic Church to obtain permission to use pontifical insignia as a mark of recognition of their previous ministry (they also may be admitted to the national or regional episcopal conference with status equivalent to that of retired Catholic bishops), but former Anglican bishops typically have not requested permission to use pontifical insignia under this provision.

Three types of mitres are worn by Roman Catholic clergy for different occasions: