The 1978 United States Senate election in Illinois took place on November 7, 1978. Incumbent Republican United States Senator Charles H. Percy ran for re-election to a third term in the United States Senate. Percy was opposed by Democratic nominee Alex Seith, an attorney who had been appointed to several local government positions.

Charles Harting Percy, also known as Chuck Percy, was an American businessman and politician. He was president of the Bell & Howell Corporation from 1949 to 1964, and served as a Republican U.S. senator from Illinois from 1967 until 1985, following a defeat to Paul Simon. He was mentioned as a Republican presidential hopeful from 1968 through 1988. During his Senate career, Percy concentrated on business and foreign relations.