Ryan has been politically battered since the release of records alleging he tried to pressure his ex-wife, actress Jeri Ryan, to perform sex acts while others watched.
Ryan issued a statement today that says he decided to pull out because the allegations had made it almost impossible to hold a "vigorous debate on the issues."
Illinois GOP leaders plan a news conference later Friday. State Republicans can now select another candidate to face Democrat Barack Obama in the November election.
Among those who have been mentioned are state Senator Steve Rauschenberger and dairy owner Jim Oberweis both of whom lost to Ryan in the primary.