The Belgium midfielder moves to the fellow Serie A club, signing a deal until 2022 after four years at the Giallorossi
Inter Milan have signed Radja Nainggolan from Roma, the club have confirmed.
The Belgian midfielder moves to Inter after making 203 Serie A appearances for Roma, who he joined from Cagliari, initially on loan, in 2014.
Nainggolan will be reunited with coach Luciano Spalletti at San Siro – the pair previously worked together in the Italian capital – ahead of the Nerazzurri's return to the Champions League.
Inter, who snatched fourth place in Serie A with a dramatic final-day victory over Lazio in May, have reportedly paid €24 million (£21m/$27m) to bring in the 30-year-old from their domestic rivals.