Fulham said on their website that the 30-year-old had signed a three-year deal for an undisclosed sum.
Riise, who spent seven years at Liverpool and was a member of their 2005 Champions League-winning team, has been at Roma for three years.
"I'm delighted to have completed my transfer to Fulham this morning and am looking forward to playing in the Premier League once again," Riise told the website.
"Fulham is a club that I have been following as my brother, Bjorn, plays here and he speaks very highly of the club and the staff.
"Obviously I also played against the club in the Europa League group stage when they went on to reach the final of the competition."
Riise became new manager Martin Jol's second signing of the close season, following the arrival of Hungarian goalkeeper Csaba Somogyi.
Jol told the website: "John Arne Riise is an experienced international left-back, who will bring a number of qualities to our squad and provide more options for me.
"He is used to performing at the very highest level for both club and country and is an accomplished player defensively, who can also contribute to our attacking play."
Riise will start training next week and will not take part in Fulham's Europa League qualifier against Crusaders in Belfast on Thursday.
