A priest or priestess is a person having the authority or power to perform and administer religious rites; in particular, rites of sacrifice to, and propitiation of, a deity or deities. Their office or position is the priesthood, a term which may also apply to such persons collectively.

Priests and priestesses have been known since the earliest of times and in the simplest societies. They exist in some branches of Christianity, Shintoism, Hinduism, and many other religions, as well, and are generally regarded as having good contact with the deities of the religion to which he or she ascribes, often interpreting the meaning of events, performing the rituals of the religion, and to whom other believers often will turn for advice on spiritual matters.

In many (but not all) religions, being a priest or priestess is a full-time assignment, ruling out any other career. In other cases it is an auxiliary role. For example in early Icelandic history the chieftains were entitled goði, a word which meant "priest", but as in the saga of Hrafnkell Freysgoði, this consisted merely of offering periodic sacrifices to the Norse gods and goddesses, and it was not a full time occupation, nor did it involve any special training or ordination. In some religions, priesthood is a position inherited in familial lines.

The term, priestess, often is used for women officiating in ancient religious temples and oracles and, in some cultures, they would have preceded priests until later traditions emerged.

Women officiating in modern Paganism, Neopagan religions such as Wicca, and various Polytheistic Reconstructionism faiths are referred to as priestesses, however, in contemporary Christian churches that ordain women, such as those of the Anglican Communion or the Christian Community, ordained women simply are called, priests.

Those officiating in Judaism, both men and women, are called Rabbi.