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.

As per 2015, it pastorally served 634,425 Catholics (83.2% of 762,240 total) on 2,078 km2 (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)