The Roman Catholic Diocese of Saint-Jean–Longueuil (Latin: Dioecesis Sancti Ioannis–Longoliensis) is a suffragan of the Metropolitan Archdiocese of Montréal in (mostly francophone) Québec, southeastern Canada.

Its cathedral episcopal see is the Cathédrale Saint-Jean-l’Évangéliste dedicated to John the Evangelist, in Saint-Jean-sur-Richelieu.

It has a Co-Cathedral: Cocathédrale Saint-Antoine-de-Padoue, dedicated to Saint Anthony of Padua, in Longueuil, Québec, and a Minor basilica: Basilique Sainte-Anne-de-Varennes, in Varennes, Québec.

• Erected on 9 June 1933, as the Diocese of Saint-Jean-de-Québec, on territory split off from the Archdiocese of Montréal, its Metropolitan.
• It was renamed on 27 February 1982 as Diocese of Saint-Jean–Longueuil / SanctiIoannis–Longolien(sis) (Latin).

As per 2015, it pastorally served 634,425 Catholics (83.2% of 762,240 total) on 2,078 km (802 sq mi) in 45 parishes and 1 mission with 89 priests (60 diocesan, 29 religious), 4 deacons, 350 lay religious (99 brothers, 251 sisters) and 4 seminarians.

(all Roman Rite native Canadians)

Suffragan Bishops of Saint-Jean-de-Québec

• Paul-Ernest-Anastase Forget (1934.05.12 – death 1955.02.03)
• Gérard-Marie Coderre (1955.02.03 – retired 1978.05.03), died 1993; succeeded as former Titular Bishop of Ægæ (1951.07.05 – 1955.02.03) and Coadjutor Bishop of Saint-Jean-de-Québec (1951.07.05 – 1955.02.03)
• Bernard Hubert (1978.05.03 – 1982.02.27 see below), previously Bishop of Saint-Jérôme (Canada) (1971.06.25 – 1977.01.27), Coadjutor Bishop of Saint-Jean-de-Québec (1977.01.27 – 1978.05.03)

Suffragan Bishops of Saint-Jean-de-Longueuil

• Bernard Hubert (see above 1982.02.27 – death 1996.02.02), also President of Canadian Conference of Catholic Bishops (1985 – 1987)
• Jacques Berthelet, Viatorians (C.S.V.) (1996.12.27 – retired 2010.10.28), also President of Canadian Conference of Catholic Bishops (2001 – 2003); previously Superior General of Clerics of Saint Viator (Viatorians) (1984 – 1986.12.19), Titular Bishop of Lamsorti (1986.12.19 – 1996.12.27) as Auxiliary Bishop of Saint-Jean–Longueuil (1986.12.19 – succession 1996.12.27)
• Lionel Gendron, P.S.S. (28 October 2010 - retired 5 November 2019), also vice-president of Canadian Conference of Catholic Bishops (2015.09.15 – 2017.09.27); previously Titular Bishop of Tagase (2006.02.11 – 2010.10.28) as Auxiliary Bishop of Archdiocese of Montréal (Canada) (2006.02.11 – 2010.10.28)
  • Auxiliary Bishop (2004.06.19 – 2020.07.19): Louis Dicaire, Titular Bishop of Thizica (1999.02.18 – 2020.07.19), previously Auxiliary Bishop of Archdiocese of Montréal (Canada) (1999.02.18 – 2004.06.19)
  • Auxiliary Bishop (2015.12.22 – 2019.11.05): Claude Hamelin, Titular Bishop of Apollonia (2015.12.22 – 2019.11.05); next Bishop here
• Claude Hamelin (5 November 2019 – present)

• Gérard-Marie Coderre (1951–1955)
• Bernard Hubert (1977–1978)

• Robert Lebel (1974–1976), appointed Bishop of Valleyfield, Québec
• Jacques Berthelet, C.S.V. (1986–1996), appointed Bishop here
• Louis Dicaire (2004–2020)
• Claude Hamelin (2015–2019), appointed Bishop here

• Raymond Poisson, appointed Auxiliary Bishop of Saint-Jérôme, Québec in 2012

• List of Catholic dioceses in Canada

• GCatholic, with Google map - data for all sections
• Diocese of Saint-Jean-Longueuil site (in French)
• "Diocese of Saint-Jean-Longueuil". Catholic-Hierarchy. Retrieved 2007-03-15.