Inter have Radja Nainggolan from Roma on a four-year deal for a fee of €38million, while Davide Santon has moved in the other direction.
The fee for Nainggolan, who joined Roma from Cagliari in 2014 and made 203 appearances for the club, could reach a maximum of €40m with add-ons.
Santon has also signed a contract running until 2022, with the Giallorossi paying €9.5m to prise him away from the Nerazzurri.
Nainggolan will be reunited with coach Luciano Spalletti at San Siro – the pair previously worked together in the Italian capital – ahead of the Inter's return to the Champions League.
Nainggolan won 30 caps for Belgium, scoring six goals, before announcing his international retirement for the second time in less than a year after missing out on the Red Devils' World Cup squad.
However, he was an integral part of the Roma team that reached the last four of the Champions League in the 2017-18 season, Eusebio Di Francesco's side rallying on home soil to sensationally knock out Barcelona at the quarter-final stage before losing 7-6 on aggregate to Liverpool in the next round.
