Celtic manager Martin O'Neill signed Valgaeren in 2000 from Roda JC for an undisclosed fee, said to be around £3.8 million. He was an instant success at Celtic and helped Celtic win a domestic treble in his first season in Glasgow. During O'Neill's first three years at Celtic, Valgaeren was first choice in a three-man defence consisting of himself, Johan Mjällby and Bobo Baldé. He played regularly in Europe, scoring in a 4–3 win at Celtic Park against Juventus in the 2001–02 Champions League campaign, and was part of the Celtic side that reached the 2003 UEFA Cup Final. However, Valgaren struggled with injuries in his later years at Celtic and did not play regularly. When Gordon Strachan took over as Celtic manager in the summer of 2005, he was released on a free transfer after five years at the club in which he made over 100 appearances for. His final appearance for the club was the 2005 Scottish Cup Final.