The former Liverpool left-back has penned a contract until 2014 at Craven Cottage and will link up with his brother Bjorn Helge in west London.
Riise has spent the past three seasons plying his trade in Serie A with Roma, having departed from Anfield in 2008.
The 30-year-old clocked up more than 230 appearances for the Merseysiders after joining them from Monaco in 2001 and the full-back admits it is great to be back in England.
"I'm delighted to have completed my transfer to Fulham this morning and am looking forward to playing in the Premier League once again," he told the club's official 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.
"I know that the club is ambitious and continues to make good progress each year, under the guidance of the chairman, Mohamed Al Fayed.
"I am looking forward to working with Martin Jol and the rest of the squad when I report to training next week."
The Norway international is recently-appointed manager Martin Jol's second signing of the summer, following the arrival of Hungarian goalkeeper Csaba Somogyi earlier this week.
"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," said Jol.
"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.
"I am extremely happy that we've been able to add him to our squad, and believe that he will prove a valuable addition as we progress."