John Gilbert Layton (July 18, 1950 – August 22, 2011) was a Canadian academic and politician who served as the leader of the New Democratic Party (NDP) from 2003 to 2011 and leader of the Official Opposition in 2011. He previously sat on Toronto City Council, occasionally holding the title of acting mayor or deputy mayor of Toronto during his tenure as city councillor. Layton was the member of Parliament (MP) for Toronto—Danforth from 2004 until his death.
The son of a Progressive Conservative cabinet minister, Layton was raised in Hudson, Quebec. He rose to prominence in Toronto municipal politics, where he was one of the most prominent left-wing voices on the city and Metropolitan Toronto councils, championing many progressive causes. In 1991, he ran for mayor, losing to June Rowlands. Returning to council, he rose to become head of the Federation of Canadian Municipalities. In 2003, he was elected leader of the NDP on the first ballot of the convention.
Under his leadership, support for the NDP increased in each election. The party's popular vote almost doubled in the 2004 election, which gave the NDP the balance of power in Paul Martin's minority government. In May 2005, the NDP supported the Liberal budget in exchange for major amendments, in what was promoted as Canada's "first NDP budget". In November of that year, Layton voted with other opposition parties to defeat the Liberal government over the findings of the Gomery Commission. The NDP saw further gains in the 2006 and 2008 elections, in which the party elected 29 and 37 MPs, respectively.
In the 2011 election, Layton led the NDP to the most successful result in the party's history, winning 103 seats—enough for the party to form the Official Opposition for the first time. Federal support for Layton and the NDP in the election was unprecedented, especially in the province of Quebec, where the party won 59 out of 75 seats.
Layton died on August 22, 2011, after being diagnosed with cancer. He was survived by his wife of 23 years, Olivia Chow, who was elected mayor of Toronto in 2023.