Inter have completed the signing of Belgian midfielder Radja Nainggolan from Serie A rivals Roma on a four-year contract for an initial fee of €38 million, the Italian clubs said on Tuesday.

Nainggolan, who retired from the Belgium national team after being omitted from Roberto Martinez's World Cup squad, scored 33 goals in 203 appearances across all competitions for Roma since joining the club in 2014.

The 30-year-old was instrumental in Roma's run to the Champions League semi-finals last season.

Inter confirmed the move on Monday with sporting director Piero Ausilio saying, "The Belgian is a real coup for Inter. He is an important player and one that has always been decisive, particularly during his years with the Giallorossi..."

Roma said that the transfer agreement also includes a sell-on clause if Nainggolan leaves Inter in the future, with the Giallorossi entitled to 10 percent of the transfer fee -- up to a maximum of €2 million.

Nainggolan will be reunited with manager Luciano Spalletti as Inter look to push on from last season's fourth-placed finish which saw them qualify for the Champions League for the first time in six seasons.

Meanwhile, Italy defender Davide Santon has moved to Roma from Inter on a four-year contract for a fee of €9.5 million.

The 27-year-old, who has eight caps for Italy, made 110 appearances for Inter across two spells at the club.

"I cannot wait to get started and show everyone what I can do," Santon said on Roma's website. 

"Coming to Roma means a lot to me, I really wanted to make the move and being here now has given me a lot of motivation."

Roma have also signed 18-year-old midfielder Nicolo Zaniolo from Inter on a five-year contract.