After Martin O'Neill became the new manager of Celtic in June 2000, he made several attempts to sign Lennon. Finally after months of protracted negotiations with Leicester, on 6 December 2000 Lennon joined the club he supported as a boy for a fee of £5.75 million. That season saw Lennon pick-up three winner's medals as Celtic swept to a domestic treble, winning the Scottish Premier League, Scottish Cup and Scottish League Cup. The next four seasons at Celtic saw Lennon win a further two league championships and two Scottish Cups. He was also an integral part of the Celtic side that reached the UEFA Cup Final in 2003, losing 3-2 to Porto after extra time.