The Canuck letter was a forged letter to the editor of the Manchester Union Leader, published February 24, 1972, two weeks before the New Hampshire primary of the 1972 United States presidential election. It implied that Senator Edmund Muskie, a candidate for the Democratic Party's presidential nomination, held prejudice against Americans of French-Canadian descent.

The letter was a successful attempt at sabotage masterminded by Donald Segretti and written by Ken W. Clawson. In a childish scrawl with poor spelling, the author of the Canuck letter claimed to have met Muskie and his staff in Florida, and to have asked Muskie how he could understand the problems of African Americans when his home state of Maine has such a small black population, to which a member of Muskie's staff was said to have responded, "Not blacks, but we have Canucks" (which the letter spells "Cannocks"); the author further claims that Muskie laughed at the remark. While an affectionate term among Canadians today, "Canuck" is a term often considered derogatory when applied to Americans of French-Canadian ancestry in New England; a significant number of New Hampshire voters were of such ancestry.

On October 10, 1972, FBI investigators revealed that the Canuck letter was part of a dirty tricks campaign against Democrats orchestrated by the Committee for the Re-Election of the President (CRP, later derisively nicknamed CREEP).

The letter's immediate effect was to compel the candidate to give a speech in front of the newspaper's offices, subsequently known as "the crying speech". The letter's indirect effect was to contribute to the implosion of Muskie's candidacy.