Louis Plamondon (born July 31, 1943) is a Canadian politician who has served as a member of Parliament (MP) since his election in 1984. As the longest-serving current member of the House of Commons, Plamondon is Dean of the House, and holds the record as Canada's longest-serving dean. He represents Bécancour—Nicolet—Saurel (formerly known as Bas-Richelieu—Nicolet—Bécancour and Richelieu) and briefly served as the 38th speaker of the House of Commons in an interim capacity in 2023. Plamondon is a member of the Bloc Québécois; he was first elected as a Progressive Conservative (PC), until leaving the PC caucus in 1990.

Plamondon has won his seat in twelve consecutive federal elections, winning twice as a Progressive Conservative before becoming a founding member of the Bloc Québécois in 1990, after which he has been re-elected ten more times. He, along with six other Bloc MPs, resigned from the Bloc's caucus to sit as an independent MP on February 28, 2018 citing conflicts with the leadership style of Martine Ouellet. He rejoined the Bloc Québécois caucus on September 17, 2018.

Plamondon was born in Saint-Raymond-de-Portneuf, Quebec and is the brother of lyricist Luc Plamondon. He has a teaching certificate from L'École normale Maurice L. Duplessis (1964), a Bachelor of Arts degree from Laval University (1968), and a B.A.An. from the University of Montreal (1976). He was a math teacher and restaurant owner before entering political life. Plamondon supported the "oui" side in Quebec's 1980 referendum on sovereignty.

Plamondon was first elected to the Canadian House of Commons in the 1984 federal election, defeating Liberal Party incumbent Jean-Louis Leduc in Richelieu. The Progressive Conservatives won a landslide majority government in this election under Brian Mulroney's leadership, and Plamondon entered parliament as a government backbencher. He was associated with the Quebec nationalist wing of his party and soon became known as a maverick. In 1986, he criticized justice minister John Crosbie for appointing an anglophone to replace the sole francophone judge on the Ontario Court of Appeal. He later criticized industry minister Sinclair Stevens for awarding a multimillion-dollar untendered contract to a shipbuilding company in Quebec City. Plamondon also expressed sympathy with fellow MP Robert Toupin, who left the Progressive Conservatives in May 1986 to sit as an independent.

Plamondon was on the socially liberal wing of the Progressive Conservative Party. He voted against a motion to reintroduce capital punishment in 1987 and later opposed efforts to restrict abortion services. He strongly supported the Mulroney government's efforts to strengthen official bilingualism and criticized dissident anglophone Tory MPs who tried to weaken the government's reforms.

Plamondon was also one of the more pro-labour members of the Tory caucus. In 1985, he promoted a partnership between the federal and Quebec governments and the Quebec Federation of Labour's Solidarity Fund. Two years later, he stood with striking letter carriers in Sorel and criticized his own government's decision to approve replacement workers. He was prominent among a group of Quebec Tory MPs who tried to reduce the party's reliance on corporate donations. Ricardo López, a right-wing Quebec Tory MP, once suggested that Plamondon would be more suited to the social democratic New Democratic Party.

Plamondon was re-elected without difficulty in the 1988 federal election, as the Progressive Conservatives won a second majority government across the country. Over the following year, he became even more strongly aligned with the Quebec nationalist wing of his party. He supported Quebec premier Robert Bourassa's use of the Canadian constitution's notwithstanding clause to prohibit outdoor English-language signs, and expressed regret that the ban was not extended to indoor signs. He also criticized D'Iberville Fortier, Canada's official languages commissioner, for suggesting that Quebec was acting in an unjust manner toward its anglophone minority.

Consistent with his nationalist views, Plamondon was a vocal supporter of the Mulroney government's proposed Meech Lake Accord on constitutional reform and opposed Jean Charest's efforts to modify the accord in early 1990. During this period, Plamondon speculated that sovereignty-association between Quebec and Canada would be "logical and reasonable" if the accord failed and added that he might become a "Quebec-only MP" in that event. When the accord was rejected in June 1990, Plamondon resigned from the Progressive Conservative caucus and informed the House of Commons that he could no longer support a united Canada.