Julien was dismissed by the New Jersey Devils in the season's waning days.
The announcement Thursday came after days of speculation that team had hired Julien.
He is the Bruins' third coach in a year. He was let go by the first-place Devils in April with three games left in the regular season and the team preparing for the playoffs.
Lewis led to the Bruins to a 35-41-6 record in his only season. He was dismissed Friday - more than two months after they missed the playoffs for the second consecutive year - and will be given another job in the franchise. Boston hasn't won a playoff series since 1999.
In his only season with New Jersey, Julien was 47-24-8. He also coached the Montreal Canadiens for three years, leading the Canadiens to the Eastern Conference semifinals in 2004 - when they beat the Bruins in the conference quarter-finals - before he was fired midway through the 2005-06 season with a 19-16-6 record.
Devils general manager Lou Lamoriello said in taking over as coach that he didn't think Julien had the team ready to make a run at the Stanley Cup. New Jersey went on to lose in the conference semifinals to Ottawa, which then lost to Anaheim in the Stanley Cup final.
Bruins general manager Peter Chiarelli has been on the job about a year and already has fired two coaches. Mike Sullivan was let go after Chiarelli, then assistant general manager of Ottawa, took over as part of a front-office shakeup.
Julien is the 17th person to coach the Bruins in 30 years. One of them, Harry Sinden, held the job twice. Julien is the second coach hired by the Bruins who was fired by New Jersey in the closing days of the regular season. Robbie Ftorek was fired by New Jersey with eight games left in the 1999-00 season, and the Devils went on to win the Stanley Cup.
Ftorek was hired by the Bruins in 2001 - Boston's third coach in seven months - and lasted less than two seasons before he was fired nine games before the 2003 playoffs.