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. He 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 contentious and brutally 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. This is the most recent election for this seat in Illinois where both candidates are deceased.