In a newspaper interview conducted through his lawyer, the Rev. Nunzio Scarano said, "I have told of episodes that could put me in danger," ANSA news service reported.
"I am trying to be stronger than the fear and nightmares that torment me, but despite my prayers, I am certain that I will die by poisoning," Scarano added.
The prelate had headed up a key Vatican accounting department until a month before he was arrested in June.
Italian police charge he conspired with a former Italian spy and a financial broker to secretly bring into the country $27 million allegedly acquired through tax evasion.
Scarano called himself "the scapegoat of a number of powerful Vatican figures." He called on Pope Francis to talk to him "at least once" before judging him.
Scarano is currently being held in a hospital.