The 1984 United States Senate election in Illinois took place on November 6, 1984. Incumbent Republican Senator Charles H. Percy ran for re-election to a fourth term in the United States Senate. Senator Percy was opposed by Democratic nominee Paul Simon, who was a United States Congressman from Illinois's 22nd congressional district. The campaign between Percy and Simon was brutal and toughly-fought, and ended up with Simon ousting Percy by fewer than 90,000 votes, which was, at the time, considered an upset. Incidentally, Percy's son-in-law Jay Rockefeller was elected Senator from West Virginia in the same election cycle.