The 23-year-old has recently been linked the likes of Manchester City, CSKA Moscow, AC Milan, Inter Milan and Napoli.
But Cellino has now revealed that Nainggolan will join Juventus in the summer for around £12.5million.
He told Tuttosport: “I have laid a pact of honour with Juventus for Nainggolan. Unless they change their minds in June then I expect Radja to be wearing the famous black and white shirt.
“I gave my word to (Juventus’ sporting director, Giuseppe) Marotta.
“Radja was enchanted by the Juventus stadium when he came to play with Cagliari.”
