The 32-year-old former Belgium international made just five appearances in all competitions for Inter Milan in the first half of the season. All those appearances came from the bench.
This will be Nainggolan’s third spell with Cagliari and he will wear the number four jersey.
Nainggolan played for the Sardinia-based club between 2010 and 2014 before departing for AS Roma, where he enjoyed another four-year spell.
He joined Inter Milan in the summer of 2018 but was sent out on loan at Cagliari for the entirety of the 2019/2020 season.
An official statement on Cagliari's website reads: "Cagliari Calcio is pleased to announce that it has acquired the right to the sports services of the player Radja Nainggolan who moves from Inter with the loan formula until the end of the season.
"Like a long love story, tinged with red and blue: Cagliari and Nainggolan once again together. Because the little boy who became a man and a footballer on the island, who left to take flight and rise to an absolute level champion, has always carried the flag of Sardinia with him, the Quattro Mori tattooed in his heart.
"Radja Nainggolan is back, ready to wear his 4 again, to battle in the middle of the field, to write new pages in Cagliari."
"I'm happy to be here," Nainggolan told reporters on Wednesday. "I want to rediscover the joy of playing football that was taken away from me in this first part of the year.
"The team is strong and competitive and with many promising young players.
"I've read many things but I can say I'm intact and can't wait to play. Will I stay permanently? I'm being moved left and right, so let's see."
Cagliari are currently 15th on the Serie A table.
