The Co-Cathedral of Saint-Antoine-de-Padoue (French: Co-cathédrale Saint-Antoine-de-Padoue) is a co-cathedral in Longueuil, Quebec, Canada, on Montreal's south shore. It is located on the corner of Rue Saint-Charles and Chemin Chambly in the Borough of Le Vieux-Longueuil. It is dedicated to St. Anthony of Padua. The cathedral houses the remains of the Blessed Marie-Rose Durocher, the foundress of the Sisters of the Holy Names of Jesus and Mary.

Its episcopal region is Longueuil-Nord. Lionel Gendron, the bishop, has a cathedra sculpted in walnut. Before the reign of Bernard Hubert, it was simply a parish church.

The cathedral was classified as historical monument by the Government of Quebec in 1984.

The site contains the archaeological remains of Fort Longueuil, a fort constructed between 1685 and 1690 as the fortified residence of Charles le Moyne de Longueuil, the only Canadian-born person to be raised to the rank of Baron by the French King. The fort was demolished in 1810 and the cathedral contains stone building materials and elements salvaged from the fort. The site of the fort was designated a National Historic Site of Canada in 1923.

The Parish of Saint-Antoine-de-Padoue was founded in 1698, and is one of the oldest in Canada. The present cathedral building was largely built from 1884 to 1887, although it was not completed until 1911. It is the third church in the history of Longueuil, the first being completed in 1811.

Saint-Antoine-de-Padoue was designated as a co-cathedral in 1982 when the Roman Catholic Diocese of Saint-Jean-de-Québec was renamed the Roman Catholic Diocese of Saint-Jean-Longueuil. The Cathedral of Saint-Jean-l'Évangéliste has been the primary cathedral of the diocese since its establishment in 1933.

A funeral was held for Jean-Pierre Côté, the 23rd Lieutenant Governor of Quebec, on July 17, 2002.

In 2005, that the faithful of the diocese paid tribute to Pope John Paul II, following his death. They also wished a happy pontificate to his successor, Pope Benedict XVI, during a special vigil attended by the bishop as well as a local congregation of Filipino Religious Sisters.

The cathedral was constructed in the Gothic revival style of architecture, while the dome is an example of Byzantine Revival architecture.

The architects Henri-Maurice Perrault and Albert Mesnard wanted the cathedral to be of great volume. The same architects built the church's altar, combining fine stones with the hardest stone. The Québécois sculptor, Louis-Philippe Hébert, contributed to the cathedral's facade, by creating three sculptures out of wood, covered in metal. The church was constructed at a cost of $98,895 by Eugène Fournier dit Préfontaine, an entrepreneur, farmer and carpenter.

The cathedral is very large, measuring 74 metres (243 ft) long, 41 metres (135 ft) wide and 81 metres (266 ft) high. Louis Jobin renovated the church in 1930. The roof, which had been covered in steel, was restored in 1999 using 60,000 pounds (27,000 kg) of copper. Further restoration work is planned.

Mass is ordinarily celebrated twice Monday to Friday, once Saturday, and four times Sunday. The Confessionals are normally ready fifteen minutes before mass. Around one third of the cathedral is full for weekend services, while it is less than one sixth full during the week. There are approximately 9,400 Roman Catholics in the parish. The churchwardens regularly organize fundraising campaigns to proceed with renovations to the cathedral as well as for the parish's rectory.

Part of the cathedral's crypt holds the graves of the Le Moyne and Grant families, affiliated with the title of Baron de Longueuil.

Inside the cathedral, there is a plaque listing all the priests in the parish's history.

Name	Years	Name	Years
Pierre Millette	1698-1701	L. Moïse Brassard	1840 -1855
Pierre de Francheville	1701-1713	Georges-Amable Thibault	1855-1883
Fr. Nic. Ber. Constantin	1713-1715	Maximilien Tassé	1883-1901
Claude Dauzaf	1715-1717	J.-Georges Payette	1901-1938
François Céré	1717-1720	Mgr. Albéric Picotte	1938-1943
Joseph Isambart	1720-1763	Mgr. Romain Boulé	1943-1962
Claude-Charles Carpentier	1763-1777	J. Alcide Careau	1963-1974
Charles-Basile Campeau	1777-1782	Jean-Louis Yelle	1974-1983
J.-Étienne Desmeules	1783-1789	Jean-Hugues Trudeau	1983-1995
Mgr. Pierre Denaut	1789-1806	Raymond Poisson	1995-2007
Augustin Chaboillez	1806-1834	Yves Le Pain	2007-
Antoine Manseau	1834-1840

• Official website of the parish (in French)
• Website of Vigilia Saint Antoine (in French)
• Cocathédrale Saint-Antoine-de-Padoue. La recherche d'une volumétrie expressive par Denyse Légaré (publié sur le site du Conseil du patrimoine religieux du Québec (in French)
• Comment on a restauré la toiture (in French)
• L'art sacré dans la cocathédrale -- Site de la société historique du Marigot Archived 2005-11-18 at the Wayback Machine (in French)