Thomas Chase-Casgrain, PC (28 July 1852 – 29 December 1916), also known as Thomas Casgrain, was a French-Canadian lawyer and politician. As a young attorney he became famous for his participation in the prosecution of Louis Riel. He was born at Detroit in 1852, the son of Charles Eusèbe Casgrain, and studied at the Petit Séminaire de Québec and the Université Laval. He was admitted to the Quebec Bar in 1877 and received the Dufferin Silver Medal in the same year. He was named professor of law at Université Laval two years later.