Royal Antwerp have snapped up released Inter Milan midfielder Arturo Vidal.
Nainggolan has joined Antwerp ahead of an offer to return to Cagliari.
“It feels good to be back," Nainggolan told Sporza reporters.
“I chose Antwerp because the chairman really wanted me and it's an interesting project. I'm confident in the project here."
Antwerp are rivals to his hometown club of Beerschot, so some are not taking the move well back in Belgium.
“This is not something I am doing against the Beerschot fans, it's a choice for myself. I've been away from home for 16 years and now I'm happy to be back in Antwerp."