Public prosecutors in Milan launched the probe after a leaked recording emerged of a meeting between a close aide to the deputy prime minister, Matteo Salvini, and Russian businessmen.
According to BuzzFeed, they discussed a Russian energy giant selling $1.5bn (£1.2bn) of oil to an Italian oil firm, buying and selling through intermediaries, to divert a sum worth $65m to Mr Salvini’s far-right League party.
Mr Salvini, who is the interior minister in a coalition-government with the populist 5-Star Movement, has denied his party received any money from Moscow.
His former spokesman, Gianluca Savoini, admitted to meeting a group of unnamed businessmen in Moscow last October but denied the League had received any funds.
Two sources with knowledge of the case told Reuters that magistrates have now opened a probe into the allegation of international corruption.
Italian law forbids political parties from accepting donations from foreign entities.
Mr Salvini is facing calls from the opposition centre-left Democratic Party to appear in parliament to address the allegations.
The speaker of the Senate, who is a member of the League’s longstanding political ally, Forza Italia, refused to summon Mr Salvini.
She has dismissed the controversy as “journalistic gossip”.
Vladimir Putin visited Rome last week and praised Mr Salvini, telling the Corriere della Sera newspaper he was “in constant contact” with the League.
“Salvini has a welcoming attitude towards our country,” the Russian president said.
Tough financing laws in Italy aimed at stamping out years of corruption have left parties struggling to fill their bank accounts.
An Italian court last year gave the League 75 years to pay back some €49m it owes the state following a trial.