Olivier Morin/AFP
"Silvio Berlusconi is under investigation in Naples for corruption in a scandal over the 'purchase' of senators," leftist daily La Repubblica said on its website.
"Prosecutors believe he paid senator De Gregorio three million euros ($3.9 million)," it said.
The case goes back to parliamentary elections in 2006 which were won by a centre-left coalition by just a handful of votes.
A few months after the elections, a senator from the winning coalition, Sergio De Gregorio, crossed the aisle and joined the Berlusconi opposition in a move that helped bring down the government in 2008.
The subsequent elections were won handily by Berlusconi.
The investigation is being handled by prosecutors in Naples since that was De Gregorio's seat.
The Corriere della Sera daily said five prosecutors including anti-mafia investigators were in charge of the probe.
The scandal-tainted Berlusconi has often been accused of buying votes or bribing lawmakers but this is the first time he is officially under investigation for allegedly corrupting a politician.
The billionaire tycoon has been a defendant in multiple court cases but so far has only one outstanding conviction for tax fraud, which he is appealing.
He is also on trial for allegedly having sex with an underage prostitute when he was still prime minister and abusing the powers of his office.
