Sir Walter Gibson Pringle Cassels (14 August 1845 – 1 March 1923) was a Canadian lawyer and judge.
He was educated at Quebec High School and the University of Toronto, graduating with a BA in 1865.
He was called to the Bar in Ontario in 1869, and practiced at Blake, Lash, Cassels.
As a judge of the Exchequer Court, he was on occasion to sit as an ad hoc judge in the Supreme Court of Canada, which he did thirty-three times from 1918 to 1922.
Cassels Lake in Temagami, Ontario is named in his honour.