Sir Alexandre Lacoste, PC (January 13, 1842 – August 17, 1923) was a Canadian lawyer, professor, and politician.

He was born in Boucherville, Canada East (now Quebec), in 1842, the son of Louis Lacoste.

In 1884, he was called to the Senate of Canada representing the senatorial division of De Lorimier, Quebec.

The couple's eldest son, Louis Joseph Lacoste, married Bertha Louisa, daughter of M. S. Foley, Esquire, editor-proprietor of the Journal of Commerce.

Marie, their eldest daughter, wrote legal text-books and married an advocate, Henri Gerin-Lajoie.