The Belgian midfielder only joined Inter in June from Roma in a deal reported to be worth €38 million (£34m/$43) plus Nicolo Zaniolo, though he has failed to live up to expectations.
Nainggolan has scored two goals in 11 Serie A outings, with Inter currently sitting third in the table.
But he looks unlikely to feature in their next few games following Inter's shock announcement that he will be barred from "football activity".
Inter's statement read: "Inter can confirm that Radja Nainggolan has been temporarily suspended from football activity for disciplinary reasons."
The news comes at the end of a difficult week for the combative midfielder, as it was reported on Thursday that a gang had been falsifying his cheques and managed to rob him of approximately €150,000.
According to reports, an investigation into the crime is then claimed to have found Nainggolan suffered significant losses at a casino.
Inter have not stated the reasons for Nainggolan's suspension, but he has long been seen as a contentious figure in Italian football.
The extravagant character referred to himself as "normal" rather than a "bad boy" in an interview last month, though he has often attracted criticism for smoking, partying and drinking.
He was left out of Belgium's World Cup squad as he failed to convince Roberto Martinez he was worthy of a place, with the pair having had a rocky relationship during the Spaniard's time in charge.
Reports in the Italian media suggest Nainggolan's punishment is for arriving late to training.