An antipope (Latin: antipapa) is a person who, in opposition to the legitimately elected pope, makes a significant attempt to occupy the position of Bishop of Rome and leader of the Catholic Church. At times between the 3rd and mid-15th centuries, antipopes were supported by important factions within the Church itself and by secular rulers.

Sometimes it was difficult to distinguish which of two claimants should be called pope and which antipope, as in the case of Pope Leo VIII and Pope Benedict V.Persons who merely claim to be pope and have few followers, such as the modern conclavist antipopes, are not classified with the historical antipopes.