Isaac Stephenson (June 18, 1829 - March 15, 1918) was an American politician of the Republican Party who represented Wisconsin as both a United States Representative and a United States Senator.
Born in York County, New Brunswick, Canada, his family later moved to Maine. Stephenson moved to Marinette, Wisconsin, in 1858 and worked in lumber. He would hold several offices including a town supervisor, county board chairman, and justice of the peace. Stephenson served in the Wisconsin State Assembly in 1866 and 1868, and three terms in the United States House of Representatives from 1883 to 1889. Stephenson returned to Congress as a U.S. Senator succeeding John Coit Spooner, who resigned. Stephenson served in the Senate from 1907 to 1915.
His younger brother, Samuel Merritt Stephenson, served as a U.S. Representative from Michigan.