"I've never held an account in Switzerland or elsewhere abroad, " Cahuzac told AFP on Tuesday in a written statement.
He said he had asked his lawyers to submit a defamation complaint as soon as possible over the report by the Mediapart website.
Known for its investigative work, Mediapart reported on Tuesday that Cahuzac "had for many years held an undeclared bank account" with Swiss banking giant UBS in Geneva.
The site said it had several sources and documents supporting the claim but did not cite them directly.
It claimed Cahuzac, a member of the Socialist party, had shut down the account in early 2010, a few days before he became president of the French national assembly's financial committee.
"The assets were then moved to another tax haven" Mediapart said, naming UBS in Singapore and citing "knowledgeable sources".
Cahuzac, 60, has been budget minister since May in the government of President François Hollande, who won election on the promise of government transparency.