John Patrick Marcus "Jack" Kavanagh (12 July 1879 – 6 July 1964) was a socialist leader in Canada and Australia. He was leader of the Socialist Party of Canada from 1908 to 1921. At the founding meeting of the Worker's Party of Canada, which was the public face of the underground Communist Party of Canada, in February 1922, he was elected to the National Executive Committee of the party. His wife's name was Kylsea, and he had one brother, Quinn.