Pierre-François Casgrain (August 4, 1886 - August 2, 1950) was a Canadian politician and Speaker of the Canadian House of Commons from 1936 to 1940.

He was born the son of a Montreal physician and raised by his grandmother following the death of his mother when he was three years old. Casgrain graduated in law from Laval University and practiced in Montreal where he worked as an organiser for the Liberal Party of Canada and the Liberal Party of Quebec. When his father-in-law Sir Rodolphe Forget , the Conservative MP for Charlevoix, retired from politics in Casgrain decided to run for the seat as a Liberal in the 1917 Canadian election. The campaign occurred as a result of the Conscription Crisis of 1917 and Casgrain ran as an opponent of the draft and was elected to the Canadian House of Commons.

From 1921 to 1925, Casgrain was the parliamentary whip of the Quebec Liberal caucus and from 1926 to 1936 he was the Chief Whip of the Liberal caucus.

Casgrain was nominated by Prime Minister William Lyon Mackenzie King to be Speaker of the House in 1936. He served in this position until 1940 when he was appointed to the Cabinet as Secretary of State for Canada a position that had sweeping emergency powers during World War II. In 1942 Casgrain was given a judicial appointment and retired from politics. He died in 1950.

Casgrain's wife, Thérèse Casgrain, was a prominent political figure in her own right.