The Diocese of Manchester (Latin: Diocensis Manchesteriensis) is a Latin Church ecclesiastical territory, or diocese, of the Catholic Church for New Hampshire in the United States. It is a suffragan diocese in the ecclesiastical province of the metropolitan Archbishop of Boston.

The mother church of the Diocese of Manchester is the Cathedral of St. Joseph in Manchester. Its patron saints are St. Joseph and St. Patrick.

As of 2018, the diocese served 322,258 Catholics (24.3% of 1,326,813 total population) on 24,097 km in 89 parishes with 185 priests (121 diocesan, 38 religious, 26 extern), 73 deacons, 314 lay religious (15 brothers, 299 sisters) and 14 seminarians.

The first Catholics in the New Hampshire area were probably members of the Sokwaki and Pennacook tribes who had been converted by missionaries from the French colony of New France. After New Hampshire became a British royal colony in 1680, it enacted discriminatory laws against Catholics, including the requirement of a Protestant oath for anyone seeking public office.

During King William's War in the late 1600s, Native American allies of the French captured several women from the British settlements in the Province of New Hampshire. These women later converted to Catholicism; one of them traveled to New France to enter an Ursuline convent. In 1694, during a French raid on British settlements near Durham, the first Catholic Masses in New Hampshire were celebrated by two Jesuit priests accompanying the expedition. However, there would be no organized Catholic communities in New Hampshire until the 19th century.

After the American Revolution ended in 1783, Pope Pius VI erected in 1784 the Prefecture Apostolic of the United States, encompassing the entire territory of the new nation. The State of New Hampshire was admitted to the union in 1788. Pius VI created the Diocese of Baltimore, the first diocese in the United States, to replace the prefecture apostolic in 1789.

Pope Pius VII erected the Diocese of Boston on April 8, 1808, including New Hampshire in its jurisdiction. The first Catholic church in New Hampshire, St. Mary's, was built in 1823 in Claremont by a father and son, both Anglican priests, who had converted to Catholicism. The first parish in New Hampshire was St. Aloyisius in Dover, erected in 1830. It was estimated in 1835 that there were 387 Catholics, two churches, and two priests in the state.

With the industrialization of New Hampshire in the 19th century, many Catholic Irish and French Canadian immigrants started settling there. In 1853, Pope Pius IX erected the Diocese of Portland in Maine, including New Hampshire in its territory. In 1858, the first Catholic grammar school in the state opened in Manchester.

Pope Leo XIII erected the Diocese of Manchester on April 15, 1884, removing New Hampshire from the Diocese of Portland. Leo XIII appointed Denis Bradley of Portland as the first bishop of Manchester. During Bradley's tenure, the Catholic population went from 45,000 to over 100,000 and the number of priests from 40 to 107. At some point in the 1880s, Bradley contacted the Benedictine monks at Saint Mary's Abbey in Newark, New Jersey, about creating a Catholic college in New Hampshire. The Benedictines opened Saint Anselm College in Goffstown in 1889.

After Bradley's death in late 1903, Pope Pius X appointed John Delany as the second bishop of Manchester. However, after only 21 months in office, Delany died in 1906. His replacement as bishop was George Guertin, appointed by Pius X in 1907. Between 1907 and 1926, Guertin added sixteen new parishes in the diocese; five were French-speaking and two were Polish-speaking. During World War I, Guertin had the diocese purchase a $15,000 war bond; using his personal funds, he purchased $5 war saving stamps for each student in the cathedral parish school. In September 1930, Guertin decreed that Catholic parents in the diocese must send their children to Catholic schools or be denied absolution.

After Guertin died in 1931, Pope Pius XI appointed Auxiliary Bishop John Peterson from the Archdiocese of Boston as the fourth bishop of Manchester. A major area of tension in the diocese came from ethnic strife between the Irish and French-Canadian communities. A French speaker, Peterson told a Manchester dinner audience in 1932 that he condemned all religious and ethnic hatred and would not support any cause based in hatred. He was able to gain the trust of French Canadian Catholics in the diocese with his words and actions. Rivier College for girls was founded by the Sisters of the Presentation of Mary in Nashua in 1933. In 1934, in the midst of the Great Depression, Peterson enacted austerity spending measures for the diocese.

With the 1944 death of Peterson, Pope Pius XII appointed Bishop Matthew Brady from the Diocese of Burlington to be bishop of Manchester. He presided over a period of unprecedented growth in the diocese, founding 27 parishes in 11 years and authorizing the construction of nearly 50 churches and numerous schools, convents, and other facilities. The number of parishioners increased by 50,000, and the number of priests and religious from around 650 to over 1,600. For all these accomplishments he was nicknamed "Brady the Builder."

Pope John XXIII appointed Ernest Primeau from the Archdiocese of Chicago as bishop of Manchester in 1958 following Brady's death that year. Primeau founded the first foreign mission of the diocese in 1963 in Cartago, Colombia. During his tenure, the number of Catholics in the diocese increased by 43,000 and the number of parishes by 11; however, weekly Mass attendance declined from over 70% to below 50%. Primeau retired as bishop in 1974.

Odore Gendron was appointed bishop of Manchester by Pope Paul VI in 1974. Gendron created a permanent diaconate in the diocese and joined the New Hampshire Council of Churches. He also founded Magdalen College in Bedford and Thomas More College of Liberal Arts in Merrimack.

In 1983, four nuns with the Sisters of Mercy settled a lawsuit against Gendron and the diocese after being fired from their teaching jobs in 1982 at Sacred Heart School in Hampton. The nuns agreed to apply for new jobs at another diocesan school and the diocese promised to help them in their applications. It was the first lawsuit by nuns against their bishop in US history.

In 1989, Pope John Paul II appointed Auxiliary bishop Leo Edward O'Neil of the Archdiocese of Springfield as coadjutor bishop in Manchester to assist Gendron.

When Gedron retired in 1990, O'Neil automatically succeeded him as bishop of Manchester. During his tenure, O'Neil worked to foster a common vision among New Hampshire Catholics with a program entitled "Renewing the Covenant." He died in 1997.

John Paul II appointed Auxiliary Bishop John McCormack of the Archdiocese of Boston as O'Neil's replacement in 1998. In April 2011, McCormack criticized proposed cuts in the New Hampshire state budget that would impact the poor. In response, Republican State Representative D.J. Bettencourt called McCormack a "pedophile pimp" with no moral authority to criticize anybody. New Hampshire Republican Party Chairman Jack Kimball immediately disavowed Bettencourt's remarks.

After McCormack retired in 2011, Pope Benedict XVI appointed Auxiliary Bishop Peter Libasci from the Diocese of Rockville Centre as bishop of Manchester that same year. In 2014, Edward Arsenault pleaded guilty to embezzling $300,000 during the 2000s from a Catholic hospital, the estate of a deceased priest and from McCormack. Arsenault spent the money on trips with a male companion. Arsenault was laicized by the Vatican in 2017.

As of 2023, Libasci is the current serving bishop.

In early 2002, Bishop McCormack announced the names of 14 diocesan priests who had been accused of sexually abusing children. In December 2002, the diocese had admitted that its failure to protect children from sexual abuse may have been a violation of criminal law, becoming the American first diocese to make such an admission. Under threat of indictment by the New Hampshire Attorney General, McCormack signed an agreement acknowledging that the Attorney General had evidence sufficient to win convictions as part of the settlement. The diocesan attorney, Ovide M. Lamontagne said that McCormack and other prominent church members wanted a speedy settlement and, in an example of behaving "pastorally" rather than as a litigant, instructed their attorneys to take a moderate stance and eschew hardline legal tactics. Lamontagne said of the diocese's legal strategy, "That is not typical in terms of client requests."

Court papers released in January 2003 showed that Bishop Gendron destroyed records of sexual abuse by two different priests during the 1980s.

• The first instance was in 1986 for Philip Petit, a diocesan priest who molested a teenager between 1979 and 1981. Petit left the priesthood in 1986 and Gendron destroyed all of his treatment records at Petit's request. Petit was laicized by the Vatican in 2018.
• The second instance happened in 1989, when the Servants of the Paraclete treatment facility in New Mexico requested that Gendron destroy the treatment records of Reverend Gordon MacRae, a diocesan priest who was a former patient. In 1994, MacRae was sentenced to 33 to 67 years in state prison for molesting children.

In 2003, the diocese reached a settlement with the Attorney General.. The settlement spared the diocese from being criminally charged. In all, in the period of 2002 to 2003, the diocese agreed to a $15.5 million settlement involving 176 claims of sex abuse. The May 2003 settlement of 61 abuse claims for $6.5 million, handled by Manchester attorney Ovide M. Lamontagne as diocese counsel, saved the diocese from criminal prosecution.

A 2003 report by the Attorney General revealed that Gendron helped a priest accused of sexual abuse avoid criminal charges. In 1975, police in Nashua, New Hampshire, had arrested Reverend Paul Aube, a diocese priest, after finding him with a boy in a car, both with their pants down. Aube, who had admitted to acts of sexual abuse in 1972, confessed his guilt to Gendron. He asked Gendron to send him for treatment and relieve him of parish duties. Instead, Gendron called the Nashua police chief to drop charges against Aube. Gendron then transferred Aube to a parish in Rochester, New Hampshire. In 1981, the mother of a 15-year-old boy discovered Aube having sex with him in the church rectory. When advised of the new allegation, Gendron did not report Aube to the police. In 2002, Aube turned himself into New Hampshire state authorities and became a cooperating witness.

In 2004, Reverend Leo Landry, a priest convicted of sexually abusing minors, described a meeting that he had with Bishop Primeau in 1967. A woman had complained to Primeau that Landry had been seen having sex with her 13-year-old son at the family's lakeside camp in Milton, New Hampshire. Primeau summoned Landry to a meeting, in which Landry confessed his guilt. Primeau told him to stay away from the boy and write a letter of apology to the family. According to Landry, he never wrote the letter and Primeau never reported him to authorities or removed him from ministry.

In July 2019, the diocese released a list of 73 priests and religious order members who were "credibly accused" of committing acts of sexual abuse. Some of those listed were criminally convicted, defrocked, removed from public ministry, or died without receiving punishment.

In July 2021, Bishop Libasci was named in a lawsuit accusing him of child molestation between 1983 and 1984 when he was parochial vicar at Saints Cyril and Methodius Parish School in New York. The accuser, then 12 or 13 years old, said that Libasci fondled his genitals on "numerous occasions", including one instance when the boy was setting up the altar for mass. Libasci denied the accusations. In August 2021, the Archdiocese of Boston announced a formal investigation into the accusations. As of August 2024, the investigation is on hold, pending the outcome of the lawsuit.

1. Denis Mary Bradley (1883–1904)
2. John Bernard Delany (1904–1906)
3. George Albert Guertin (1906–1931)
4. John Bertram Peterson (1932–1944)
5. Matthew Francis Brady (1944–1959)
6. Ernest John Primeau (1960–1974)
7. Odore Joseph Gendron (1974–1990)
8. Leo Edward O'Neil (1990–1997; coadjutor bishop 1989–1990)
9. John Brendan McCormack (1998–2011)
10. Peter Anthony Libasci (2011–present)

• Robert Edward Mulvee (1977–1985), appointed Bishop of Wilmington and later Coadjutor Bishop of Providence, subsequently succeeding to bishop
• Joseph John Gerry, O.S.B. (1986–1988), appointed Bishop of Portland
• Francis Joseph Christian (1996–2018)

Thomas Michael O'Leary, appointed Bishop of Springfield in Massachusetts in 1921

• Wilfred J. Lessard (c. 1926)
• William Collins (1940–1948)
• George Murray (1960-1972)
• Joseph P. Duffy (1972–1975)
• Thomas S. Hansberry (1975–1976) Interim
• George J. Soberick (1976–1981)
• Roger Lemoyne (1981–1990)
• Joachim Froehlich (1990–1991)
• William T. Garland (1991–1996)
• Mary Moran (2006–2012)
• Dennis J. Audet (2012–2013) Interim
• Rev. John R. Fortin, OSB (2013–2016)
• David Thibault (2016- )

• Bishop Brady High School – Concord
• Bishop Guertin High School – Nashua
• Holy Family Academy* – Manchester
• Mount Royal Academy – Sunapee
• St. Thomas Aquinas High School – Dover
• Trinity High School – Manchester

• Magdalen College of the Liberal Arts* – Warner
• Rivier University* – Nashua
• Saint Anselm College* – Goffstown
• Thomas More College of Liberal Arts* – Merrimack

* Schools operated independent of the Diocese

• Catholic Church by country
• Catholic Church in the United States
• Ecclesiastical Province of Boston
• List of Catholic archdioceses (by country and continent)
• List of Catholic dioceses (alphabetical) (including archdioceses)
• List of Catholic dioceses (structured view) (including archdioceses)
• List of Catholic dioceses in the United States

• Official website
• Diocese of Manchester at Catholic-Hierarchy
• Diocese of Manchester at GCatholic.org
• Herbermann, Charles, ed. (1913). "Diocese of Manchester" . Catholic Encyclopedia. New York: Robert Appleton Company.
• Parable - diocesan magazine