Myrtle Agnes Cain (April 11, 1894 – February 6, 1980) was an American politician and labor activist.
Born in Minneapolis, Minnesota, Cain went to the Minneapolis public schools and to the St. Anthony's Convent. Cain served as president of the Women's Trade Union of Minneapolis and was a member of the Minnesota Farmer Labor Party. In 1923 and 1924, Cain served in the Minnesota House of Representatives. Later, she served on the staff of United States Senator Eugene McCarthy of Minnesota. Cain died in Minneapolis, Minnesota.