An antipope is one whose claim to being Pope is the result of a disputed or contested election. These antipopes were usually in opposition to a specific person chosen by the papal electors (since the Middle Ages, the college of cardinals). Some self-appointed leaders of smaller churches are also called "antipopes."

During certain periods of turbulence in the Roman Catholic Church, controversial Papal elections were conducted. Some such elections were considered invalid, either because a large majority of papal electors claimed the election was invalid (such as the election of Felix V), or because they have subsequently been declared invalid (such as Clement VII).

The earliest antipope, Hippolytus, was elected in protest against Pope Callixtus I by a schismatic group in the city of Rome in the 3rd century. Hippolytus was exiled to the mines on the island of Sardinia in the company of Callixtus' successor Pope Pontian, and was reconciled to the Catholic Church before his death.

The late 14th and early 15th century saw a series of rival popes elected, one line of which is counted by the Roman Catholic Church as popes and the other as antipopes. The scandal of multiple claimants added to the demands for reform that produced the Protestant Reformation at the turn of the 16th century. (See Western Schism, Antipope Benedict XIII.)

It would not necessarily have been evident, during periods when two (or three) rival claimants existed, which was the antipope, and which was the pope, and the clear-cut distinctions made between them in retrospect can give a false sense that certainty existed among their contemporaries. Supporters might offer assistance to a given candidate, but could not know which would be determined to have been an antipope, and which the pope, until events had run their course.

There has not been an antipope since 1449, unless sedevacantist antipopes are counted (see below). Other schisms like the Church of England are controlled by lay sovereigns who do not want to have an ecclesiastical rival or begin like the Old Catholic Church and the Chinese Patriotic Catholic Association in a rejection of a primary dogma of the papacy.

Today the act of becoming an Antipope is considered a schismatic act by the Roman Catholic Church. This would result in automatic excommunication for the person who became Antipope.

List of antipopes

1. St. Hippolytus (reconciled with Pope St. Pontian and died as martyr to the church), 217–235
2. Novatian, 251–258
3. Felix II (confused with a martyr with the same name and thus considered an authentic pope until recently), 355–365
4. Ursicinus (Ursinus), 366–367
5. Eulalius, 418–419
6. Laurentius, 498–499, 501–506
7. Dioscorus (legitimate perhaps as opposed to Boniface II but died 22 days after election), 530
8. Theodore (opposed to antipope Paschal), 687
9. Paschal (opposed to antipope Theodore), 687
10. Theofylact, 757
11. Constantine II, 767–768
12. Philip (replaced antipope Constantine II briefly; reigned for a day and then returned to his monastery), 768
13. John, 844
14. Anastasius III Bibliothecarius, 855
15. Joan, 855–858 mythical female popess who supposedly reigned under the name John VIII until her sex was discovered.
16. Christopher, 903–904
17. Boniface VII, 974, 984–985
18. John Filagatto (John XVI), 997–998
19. Gregory VI, 1012
20. Sylvester III, 1045
21. John Mincius (Benedict X), 1058–1059
22. Pietro Cadalus (Honorius II), 1061–1064
23. Guibert of Ravenna (Clement III), 1080 & 1084–1100
24. Theodoric, 1100–1101
25. Adalbert, 1101
26. Maginulf (Sylvester IV), 1105–1111
27. Maurice Burdanus (Gregory VIII), 1118–1121
28. Thebaldus Buccapecuc (Celestine II ) (legitimate but submitted to opposing pope, Honorius II and afterwards considered an antipope), 1124
29. Pietro Pierleoni (Anacletus II), 1130–1138
30. Gregorio Conti (Victor IV), 1138
31. Ottavio di Montecelio (Victor IV), 1159–1164
32. Guido di Crema (Paschal III ), 1164–1168
33. Giovanni of Struma (Callixtus III), 1168–1178
34. Lanzo of Sezza (Innocent III ), 1179–1180
35. Pietro Rainalducci (Nicholas V), antipope in Rome, 1328–1330
36. Robert of Geneva (Clement VII), antipope of the Avignon line, 20 September 1378 – 16 September 1394
37. Pedro de Luna (Benedict XIII), antipope of the Avignon line, 1394–1423
38. Pietro Philarghi Alexander V, antipope of the Pisan line, 1409–1410
39. Baldasssare Cosa John XXIII, antipope of the Pisan line, 1410–1415
40. Gil Sanchez Munoz (Clement VIII), antipope of the Avignon line, 1423–1429
41. Bernard Garnier (the first Benedict XIV), antipope of the Avignon line, 1425–1430?
42. Jean Carrier (the second Benedict XIV), antipope of the Avignon line, 1430–?
43. Duke Amadeus VIII of Savoy (Felix V), 5 November 1439 – 7 April 1449

Sedevacantist antipopes

Some breakaway Catholics today, called sedevacantists, claim the current Popes are heretics for replacing the Tridentine Latin Mass with the Novus Ordo Missae; many of them also object to the celebration of the Mass in the vernacular. Since the opinion of many Catholic theologians is that a heretical Pope would cease to be Catholic and therefore cease to be Pope, sedevacantists believe the current Bishops of Rome are not actually popes. Some sedevacantist groups have their own popes to replace the popes they reject. They are sometimes called antipopes, although it should be noted that in contrast to historical antipopes, the number of their followers is minuscule.

Sedevacantist antipopes frequently refer to the conventional successors of Pope Pius XII - Popes John XXIII, Paul VI, John Paul I, John Paul II and Benedict XVI - as a series of antipapacies.

Sedevacantist antipopes of 20-21st centuries

• Michel Colin (Clement XV), self-proclaimed from 1950–1968 in Canada
• (Jean) Gaston Tremblay (Gregory XVII), succeeded Clement XV in 1968 in Canada; not to be confused with the Canadian politician Gaston Tremblay
• Gino Frediani (Emmanuel), self-proclaimed from 1973–1984 in Italy
• Clemente Domínguez y Gómez (Gregory XVII), self-proclaimed from 1978–2005 in Spain
• Francis Konrad Schuckardt (Hadrian VII), self-proclaimed in 1984 in Washington, United States of America
• Valeriano Vestini (Valeriano), self-proclaimed in 1990 in Chieti, Italy
• David Bawden (Michael), self-proclaimed in 1990 in Kansas, United States of America
• Victor Von Pentz (Linus II), self-proclaimed in 1994 in the United Kingdom
• Maurice Achieri of Le Perreux (Peter II), self-proclaimed in 1995 in France
• Lucian Pulvermacher (Pius XIII), self-proclaimed in 1998 in Montana, United States of America