The Italian side originally co-owned the midfielder with Cagliari, but has now purchased the remaining stake in the player.
Roma has announced that the club has purchased full ownership of Radja Nainggolan for 9 million euros.
The Belgium international moved to the capital club in Jan. 2014 on an initial loan deal before the side purchased 50 percent of his contract that summer.
However, after another bright season for Rudi Garcia's men, after which he attracted interest from several European sides, Roma has moved to acquire all of the rights to the 27-year-old.
"Roma has reached an agreement with Cagliari Calcio to resolve the co-ownership of footballer Radja Nainggolan in Roma's favor, in exchange for a fee of €9 million," a statement on the club's website read.