Sedevacantism is a traditionalist Catholic movement which holds that since the 1958 death of Pius XII the occupiers of the Holy See are not valid popes due to their espousal of one or more heresies and that, for lack of a valid pope, the See of Rome is vacant. Sedevacantism owes its origins to the rejection of the theological and disciplinary changes implemented following the Second Vatican Council (1962–1965).

The term sedevacantism is derived from the Latin phrase sede vacante, which means "the chair [of the Bishop of Rome] being vacant". The phrase is commonly used to refer specifically to a vacancy of the Holy See which takes place from the pope's death or renunciation to the election of his successor.

The number of sedevacantists is unknown and difficult to measure; estimates range from tens of thousands to hundreds of thousands. Various factions of conclavists among sedevacantists have proceeded to end the perceived vacancy in the Holy See by electing their own pope.

The term sedevacantism derives from the Latin term sede vacante, which means "with the chair being vacant". In the Catholic Church, when an episcopal see becomes vacant due to the death or removal of a Bishop from office for whatever reason, in the interim the diocese is automatically in a state of sede vacante, until a new designate is appointed and duly elevated to his see. With Sedevacantism, this is specifically in reference to the See of Saint Peter, i.e., the Catholic Papacy.

The Sedevacantist thesis that the post-Second Vatican Council claimants to the Papacy operating out of the Vatican City are non-Catholic Antipopes, originated from a 1973 work, Sede Vacante: Paul VI is Not a Legitimate Pope, by the Mexican Jesuit priest Joaquín Sáenz Arriaga. However, there were some instances of proto-sedevacatism, avant la lettre, reaching back into the 1960s.

Sedevacantism, avant la lettre, is evidenced from the mid-1960s, as part of a response to the Second Vatican Council in the Roman Catholic Church. The earliest example is from a group of traditionalist Catholics in Mexico associated with the radical right secret society Los TECOS based in Guadalajara, in particular their spiritual director, Fr. Joaquín Sáenz y Arriaga, a Jesuit priest. In 1965, at a private meeting in the house of Anacleto González Guerrero (son of the Cristero martyr Anacleto González Flores), Los TECOS leaders proposed the motion that Paul VI (Giovanni Montini) was a crypto-Jew and an illegitimate Pope, and that this line should be officially adopted as the position of Mexican traditionalists.

A connected secret society, based in Puebla under Ramón Plata Moreno, known as El Yunque, although ultra-conservative as well and unhappy about the liberalising changes in the Catholic world, rejected the proposal of Los TECOS, stating that Pope Paul VI was indeed the legitimate Pope of the Catholic Church. This led to a deadly split in the Mexican traditionalist scene.

Earlier, during the Second Vatican Council, Los TECOS had distributed a document entitled Complotto contro la Chiesa ("The Plot Against the Church") under the pseudonym of Maurice Pinay, warning Council fathers of a supposed "Judeo-Masonic-Communist" plot to infiltrate and destroy Christianity and the Catholic Church.