The Sardinian side announced the move Monday, with Nainggolan returning to the club where he spent four seasons prior to joining Roma in 2014.
"It was not hard to choose Cagliari, although I did have other offers," Nainggolan said in a statement, according to NBC Sports. "I spoke with some other managers but it was right to come back here and I must first thank the president.
"After our first call, he completely convinced me right away, he made promises and he has kept them. Things have gone the way I would have liked them to have gone and it was easy to reach an agreement," Nainggolan added.
Nainggolan, 31, failed to find his footing at Inter following a €38-million transfer from Roma in June 2018. The former Belgian international made 22 Serie A starts amid a series of injuries.
In July, Inter chief executive Giuseppe Marotta signaled an exit for the tough tackler when he conceded that Nainggolan and embattled striker Mauro Icardi were "not part of our project."