The former Liverpool fullback said: "I spoke to John and he said good things about the atmosphere.
Riise also remarked: "I wanted a new experience and to get to know a new type of football.
"I chose Roma because they are a great club with great fans and they won the Coppa Italia.
"I met the Coach and I will listen to him and try to give my best. The passion of Luciano Spalletti struck me and now I will try and learn Italian so that I can get to work.
"I'm the kind of player who gives everything he has for the team and I want to thank my new boss for wanting me here.
"Then there's Daniele De Rossi, who is one of the best midfielders in Europe and who is growing into a truly great player."