John Gilbert "Jack" Layton, PC, MP (born July 18, 1950) is a Canadian social democratic politician and since 2003 has been leader of Canada's New Democratic Party. He is a former member of the Toronto City Council and acting deputy mayor of Toronto, Ontario. On June 28, 2004, he was elected Member of Parliament for the constituency of Toronto—Danforth. He is married to fellow MP Olivia Chow.

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 city and metro council, and was also a Board member for the Toronto Port Authority. In 1991 he ran for mayor, but lost to June Rowlands. Remaining on council he rose to become head of the Federation of Canadian Municipalities. In 2003 he was elected head of the NDP on the first ballot of the convention.

Under his leadership, the NDP considerably increased their support, almost doubling the party's popular vote in the 2004 election, though vote splitting with the Liberals limited their gain in seats. Layton's NDP held balance of power in the Paul Martin's minority government, where 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 worked with other opposition parties in bringing down 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 won more seats than it had since its 1980s peak. The NDP's current tally of 37 MPs under Layton is just 6 seats short of the party's all-time high under Ed Broadbent.