The in-demand midfielder has joined on loan from Cagliari for 3 million euros with the option to spend another 6 million euros at the season's end to buy.
Both clubs confirmed the initial 3 million euro deal - which was considered a certainty as the Giallorossi trumped interest from potential suitors such as Napoli - on Tuesday. Should Roma choose to extend Nainngolan's stay beyond this campaign, it will cost an extra 6 million euros to buy half of the 25-year-old Belgium international's contract.
"For a Sardinian, it is difficult to leave the island," Nainggolan told Cagliari's official website. "It is for me, now that I have become Sardinian adoption. On the one hand I'm happy because the transfer may be an important step in my football career, on the other hand I am sorry because I am leaving the place where I grew up as a man and as a footballer.
"I was convinced that this club we could do something important, but unfortunately there were issues that stopped us from realizing such a project. I hope that this situation can be resolved as soon as possible because the city of Cagliari deserves much more than they have received in the past three years."