Richard Nelson Williamson (8 March 1940 – 29 January 2025) was an English traditionalist Catholic bishop, conspiracy theorist and Holocaust denier who was twice excommunicated from the Catholic Church. He was for many years a member of the Society of Saint Pius X (SSPX).

Williamson was born in London. He opposed the changes in the Church brought about by the Second Vatican Council. In 1988, he was one of four SSPX priests consecrated as bishops by Archbishop Marcel Lefebvre, for which Pope John Paul II declared all parties had incurred ipso facto automatic excommunication. The validity of the excommunication has always been denied by the SSPX, who, citing canon law, argue that the consecrations were permissible due to a crisis in the Catholic Church. The excommunications, including that of Williamson, were lifted on 21 January 2009, but a suspension from ministry remained in force.

Immediately afterward, Swedish television broadcast an interview recorded earlier at the SSPX seminary in Zaitzkofen, Bavaria. Therein, Williamson expressed his belief that no more than 200,000 to 300,000 Jews were killed during the Holocaust and that Nazi Germany did not use gas chambers. Based upon these statements, he was charged with and convicted of Holocaust denial by the district court of Regensburg, Germany. The Holy See declared that Pope Benedict XVI had been unaware of Williamson's views when he lifted his excommunication, and that Williamson would remain suspended until he unequivocally and publicly distanced himself from his stated position. In 2010, Williamson was convicted of incitement in a German court in relation to those views; the conviction was later vacated on appeal. He was convicted again in a retrial in early 2013. Williamson appealed again, but his appeal was rejected.

After a number of incidents—including calling for the resignation of Bernard Fellay as superior general of the SSPX, refusal to stop publishing his weekly email newsletter, and an unauthorised visit to Brazil—Williamson was expelled from the SSPX in 2012. Afterwards, Williamson consecrated Jean-Michel Faure [de], Tomás de Aquino Ferreira da Costa [pt], and Gerardo Zendejas [pl] as bishops in 2015, 2016, and 2017. Upon the first of the three consecrations, he was again automatically excommunicated from the Catholic Church.

Richard Nelson Williamson was born on 8 March 1940 in Hampstead, London, England. He was the second of three sons born to Helen Nelson, a Paris-born woman with American parents, and her husband John Blackburn Williamson, a manager at Marks & Spencer. He attended Downsend School in Surrey before winning a scholarship to Winchester College. His family temporarily relocated to Leicestershire during the London Blitz.^{[citation needed]} He then studied at Clare College, Cambridge, graduating in 1961 with a degree in English literature. Upon graduating, he worked as a journalist in Wales, and then returned to teach at his old school, Downsend, in 1963. He subsequently went to Ghana, where he also taught. When he returned to England in 1965, Williamson taught at St Paul's School in London.

Williamson, raised nominally Anglican, converted to the Catholic Church in 1971. After a few months as a postulant with the Oratorians of Brompton Oratory, he left. He became a member of the Society of Saint Pius X, a traditionalist Catholic faction founded in 1970 by Archbishop Marcel Lefebvre in protest against what Lefebvre saw as the liberalism of the Second Vatican Council. In common with other traditionalists, Williamson opposed the changes in the Catholic Church following the Second Vatican Council. He saw the changes as being unacceptably liberal and modernistic, and as being destructive to the Church. Among the changes he opposed were the Church's increased openness to other Christian denominations and other religions, and changes in the forms of Catholic worship such as the general replacement of the Tridentine Mass with the Mass of Paul VI. Williamson criticised Pope John Paul II, to whom he attributed a "weak grasp of Catholicism". Williamson held that the SSPX was not schismatic, but rather was composed of true Catholics who were keeping the "complete Roman Catholic apostolic faith".

Williamson entered the International Seminary of Saint Pius X at Écône, Switzerland, and in 1976 he was ordained a priest by Lefebvre along with 12 other priests and 13 sub-deacons. This was against Vatican orders, and led to Lefebvre being suspended a divinis by the Vatican. Williamson subsequently moved to the United States, where he was the rector of St. Thomas Aquinas Seminary in Ridgefield, Connecticut from 1983 when he was appointed to replace Donald Sanborn, and continued in the position when the seminary moved to Winona, Minnesota in 1988, serving until 2003.

In June 1988, Archbishop Marcel Lefebvre announced his intention to consecrate Williamson and three other priests (Bernard Fellay, Bernard Tissier de Mallerais, and Alfonso de Galarreta) as bishops. On 17 June 1988, Cardinal Bernardin Gantin, prefect of the Congregation for Bishops, sent the four priests a formal canonical warning that they would automatically incur the penalty of excommunication if they were to be consecrated by Lefebvre without the date of papal permission. Williamson and the three other priests were nonetheless consecrated bishop on 30 June 1988 by Lefebvre and Antônio de Castro Mayer. The next day, Cardinal Bernardin Gantin issued a declaration stating that Lefebvre, de Castro Mayer, Williamson, and the three other newly ordained bishops "have incurred ipso facto the excommunication latae sententiae reserved to the Apostolic See".^{[citation needed]} On 2 July 1988, Pope John Paul II issued the motu proprio Ecclesia Dei, in which he reaffirmed the excommunication and described the consecration as an act of "disobedience to the Roman pontiff in a very grave matter and of supreme importance for the unity of the Church", and that "such disobedience – which implies in practice the rejection of the Roman primacy — constitutes a schismatic act".