[13] The following seats became vacant in the nine-month period prior to the fixed election date of October 20, 2025 and will remain vacant until the next general election: The writ for a by-election must be dropped no sooner than 11 days and no later than 180 days after the chief electoral officer is officially notified of a vacancy via a warrant issued by the Speaker.

[26] As well, the Rhinoceros Party organized a protest against the Trudeau government's abandonment of electoral reform in 2017 by running thirty-two[citation needed] independent candidates, breaking their own record for most candidates nominated in a single riding in Canada, previously set in the riding of Saint Boniface—Saint Vital in the 2021 Canadian federal election.

[27] The riding of Winnipeg South Centre was vacated on December 12, 2022, following the death of Liberal MP and former cabinet minister Jim Carr.

[4] Carr had represented the riding since 2015, when he defeated Conservative incumbent Joyce Bateman, and had been battling multiple myeloma and kidney failure since 2019.

[30][31][32] Winnipeg city councillor Sherri Rollins briefly ran for the nomination before withdrawing and throwing her support behind Carr.

[5] Arpan Khanna, the party's national outreach chair and 2019 candidate in Brampton North[37] defeated Woodstock city-county councillor Deb Tait, MacKenzie's daughter and former ministerial staffer Rick Roth for the Conservative nomination.

[38][39] Gerrit Van Dorland, executive assistant to Cypress Hills—Grasslands MP Jeremy Patzer was running for the nomination until he was disqualified by the Conservatives over a dispute about whether he disclosed information to the party.

[43][44] Citing concerns with the Conservative nomination process, previous MP Dave MacKenzie endorsed Hilderley in April 2023.

[43] Western University professor Cody Groat defeated Matthew Chambers, the party candidate for the riding in the 2019 and 2021 elections for the NDP nomination.

Bergen's former campaign manager Branden Leslie defeated Rejeanne Caron, the party's 2019 candidate in Saint Boniface—Saint Vital and 2021 candidate in Elmwood—Transcona; Winkler resident Don Cruickshank,[49] Morden-Winkler MLA and former Progressive Conservative Party of Manitoba cabinet minister Cameron Friesen;[50] and Lawrence Toet, the MP for Elmwood—Transcona from 2011 to 2015 for the Conservative nomination.

[53][54][55] Maxime Bernier, who is the former MP Beauce and the current leader of the People's Party of Canada, announced on May 12, 2023, that he would run for the seat.

[61][62] Gainey won the Liberal nomination on May 15, 2023, defeating Fred Headon, vice president and general counsel of Air Canada, and 2021 La Pointe-de-l'Île candidate Jonas Fadeu.

[64] The riding of Calgary Heritage was vacated on December 31, 2022, following the October 20 announcement from Conservative MP Bob Benzen that he would resign his seat by the end of the year in order to return to the private sector.

Benzen had held the seat since a 2017 by-election in which he was elected to replace former Prime Minister and former Conservative leader Stephen Harper.

[70] The riding of Durham, represented by former Conservative leader Erin O'Toole, was vacated on August 1, 2023, following his resignation and retirement from politics.

[72][10] O'Toole, who led the party from 2020 to 2022 and served as Minister of Veterans Affairs in the government of Stephen Harper, has held the seat since a 2012 by-election.

[84][85] Other prospective candidates for the Liberal nomination who declined to run, included former Toronto city councillor Josh Colle; former Ontario MPP Eric Hoskins, who represented the area provincially from 2009 to 2018 and previously served in the provincial cabinets of Dalton McGuinty and Kathleen Wynne, including as Minister of Health and Long-Term Care from 2014 to 2018; and Toronto city councillor Josh Matlow, who has represented the area municipally since 2010; Matlow ultimately decided not to run.

She works as the Director of Programs at the Institute for Change Leaders, an organization that was founded by Toronto mayor, Olivia Chow.

[93] On May 24, the Green Party announced that Christian Cullis, a constituent coordinator for Ward 11 city councillor Dianne Saxe, would be their candidate.

[98] Lametti, who previously served as Minister of Justice and Attorney General in the government of Justin Trudeau, won the seat in 2015.

On July 19, Montreal city councillor Laura Palestini was selected by the Liberals as their candidate despite others seeking the nomination[99] such as Eddy Kara, a political strategist,[100] Christopher Baenninger, Quebec Liberal candidate in Sainte-Marie–Saint-Jacques in 2022 and Saint-Henri–Sainte-Anne in 2023,[101] and Lori Morrison, Electoral Division 1 Commissioner of the Lester B. Pearson School Board.

[101] On March 28, Craig Sauvé, independent city councillor for the district of Saint-Henri—Little-Burgundy—Pointe-Saint-Charles announced that he was standing for nomination for the New Democratic Party's candidate.

[104] The Bloc Québécois candidate will be Louis-Philippe Sauvé, the communications and administration coordinator at the Institute for Research in Contemporary Economics.

[125] The riding of Cloverdale—Langley City was vacated on May 31, 2024, upon the resignation of Liberal MP John Aldag to successfully seek the BC NDP nomination for Langley-Abbotsford in the 2024 British Columbia general election.

[139] The riding of Honoré-Mercier was vacated on January 20, 2025, upon the resignation of Liberal-turned-Independent MP Pablo Rodriguez to run in the 2025 Quebec Liberal Party leadership election.