A deal to take the Belgian midfielder back to familiar surroundings has been confirmed after weeks of speculation.
The 31-year-old was expected to be on the move this summer despite spending just one year at San Siro.
He was informed that he did not figure in the long-term plans of new Nerazzurri boss Antonio Conte.
With the exit door open, several options were considered by Nainggolan.
Cagliari were, however, the obvious choice on sporting and personal levels.
Nainggolan admitted to reporters before a switch was announced that he was pleased to be heading back to a side he boasts strong ties to.
He said: “I am happy, there is great enthusiasm.
“I always wanted to return but I did not think the return would come this year, but I am happy that is has happened. It was not hard to choose Cagliari although I did have other offers.”
He added on the back of discussions with Cagliari president Tomasso Giulini: “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 first linked up with Cagliari back in 2010, initially on loan from Piacenza.
That move was made permanent and he would spend four years with the club before heading to Roma.
Over 130 appearances were taken in, with a standing established within the Belgium national set-up.
Nainggolan will now get the chance to add to that tally of outings for the Serie A club.
He will also get the chance to return his wife Claudia Lai to her roots as she undergoes cancer treatment.
She is from Cagliari, the capital city of Sardinia, and has spoken of her “fears and nightmares” after starting chemotherapy.