Inter have signed Radja Nainggolan from Roma for a fee of €38million, while Davide Santon and Nicolo Zaniolo have moved in the other direction for a combined €14m.
The fee for Nainggolan, who signed a four-year contract, could reach a maximum of €40m with add-ons, but Inter's initial net spend stands at €24m.
Santon signed a contract with Roma until 2022, while Zaniolo has put pen to paper on a five-year deal. They cost €9.5m and €4.5m respectively, though the Nerazzurri will receive 15 per cent of any future transfer fee for the latter.
Nainggolan, who joined Roma from Cagliari in 2014 and made 203 appearances for the club, scoring 33 goals, 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 next term.
The midfielder 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.
Santon and Zaniolo will hope to play a role in a similarly impressive campaign for the Giallorossi, who finished third in Serie A, one place above Inter.
In his first Roma interview, Santon said: "I cannot wait to get started and show everyone what I can give.
"The arrival in Rome represents a lot for me, I have a great desire for redemption and having arrived here gives me so many motivations."
Zaniolo, labelled "one of the most interesting prospects in Italian football" by sporting director Monchi, added: "I'm proud to get the opportunity to represent this club.
"Now I have to demonstrate out on the pitch exactly what I can do."
Roma completed the signing of Justin Kluivert from Ajax for an initial €17.25m last week and are closing in on a deal for Javier Pastore, who underwent his medical on Tuesday ahead of prospective switch from Paris Saint-Germain.