La Stampa daily said the player is involved in a criminal probe targeting users of illegal online betting platforms, along with other unnamed suspects.
"I confirm the news of the investigation," Turin Chief Prosecutor Enrica Gabetta said in an emailed statement, without elaborating.
The newspaper did not say what kind of betting Fagioli was involved in. Italian soccer authorities ban players from betting on soccer matches but not from other types of gambling.
The prosecutor's office of the Italian soccer federation (FIGC) is also looking into Fagioli's case, a source close to the matter said, confirming another part of the La Stampa report.
A player found to have bet on soccer matches risks being disqualified for at least three years and fined at least 25,000 euros under the FIGC's code of conduct.
Juventus declined to comment on the affair. Attempts to reach Fagioli via social media messages were not immediately successful.
Fagioli has played in six of Juve's eight Serie A matches this season. Last November he made his debut for the Italian national team, coming on as a substitute in a 3-1 away friendly win against Albania.
