Nainggolan, who has impressed in Italy's top flight and helped Roma reach the semi finals of the UEFA Champions League during the 2017-18 season, joins Stefan de Vrij and Roberto Gagliardini (following a successful loan spell) as the third of Inter's permanent signings this summer.
A post on Inter's website reads: "
"Now, having fully matured as a player, it’s time for the Ninja to approach a new and exciting challenge in an Inter shirt. Welcome to the city of Milan, Radja! "
The 30-year-old has frequently been linked with moves away from Roma due to his excellent form over the past few campaigns and now joins an Inter side looking to reestablish themselves as a major European power after successfully qualifying for the Champions League.
Nainggolan, who was left out of Roberto Martinez's 23-man Belgium squad for the 2018 World Cup, scored 33 goals in 201 appearances for Roma and previously plied his trade for fellow Italian outfits Cagliari and Piacenza.
Inter will be looking to improve upon their fourth place finish in the 2017/18 season, during which they secured Champions League football for next term with a breathtaking 3-2 win at Lazio on the final day of the campaign.