Isaac Newton Arnold (November 30, 1815, Hartwick, New York - April 24, 1884, Chicago) was an attorney, American politician, and biographer who made his career in Chicago. He served two terms in the United States House of Representatives (1860-1864) and in 1864 introduced the first resolution in Congress proposing a constitutional amendment to abolish slavery in the United States. After returning to Chicago in 1866, he practiced law and wrote biographies of Abraham Lincoln and Benedict Arnold.