In the history of Christianity, the Conciliar movement or "Conciliarism" was a reform movement in the 14th and 15th century Catholic Church that held that final authority in spiritual matters resided with a general church council, not with the pope. The movement emerged in response to the Avignon papacy— the popes removed from Rome and subject to pressures from the kings of France— and the ensuing schism that inspired the summoning of the Council of Constance (1414-1418). The eventual victor in the conflict was the institution of the Papacy, confirmed by the condemnation of conciliarism at the Fifth Lateran Council, 1512-17. The final gesture however, the doctrine of Papal Infallibility, was not promulgated until 1870.

Within a century the Roman Catholic Church split over irreconcilable differences, such as the relative stress placed on Scripture and Authority where they appear to conflict and other divisive issues raised by the Conciliar movement, which is condemned in retrospect by traditionalist Roman Catholics, who support the Papacy in issues of authority.

The word "Conciliarism" is used when subtexts of heterodoxy or heresy are to be subtly emphasized, and aspects of structural reform within the Roman church are to be downplayed. Secular historians tend to use the more neutral expression "Conciliar movement," which offers no such inherent connotations of being an ideologically driven "-ism".

While not involved in the Conciliar movement of the 14th and 15th centuries, the Eastern Orthodox Church generally agrees with the conciliarists that final authority resides with the church councils rather than with the pope. This was one of the issues that led to the schism between Eastern Orthodoxy and Roman Catholicism that was finalized in 1054.