The paper said police suspect the man being held was aware of Kerviel's activities and that he was taken into custody after a search of his company offices on Thursday.
French police refused to comment, and spokespersons for the bank did not immediately return calls seeking comment on the Le Monde report.
But Societe Generale has said it believes that Kerviel had no accomplices.
Held by police
Kerviel was in custody for two days last month, and judges filed preliminary charges against him but released him during the investigation.
Joelle Rosello, a Societe Generale spokeswoman, confirmed that an employee of Newedge, a 50-per cent owned affiliate, was in police custody in the Kerviel investigation.
"We are co-operating closely with police," said Rosello.
Newedge was formed last month in a merger between Fimat and a division of Calyon, an investment bank.
Le Monde said new evidence provided by Societe Generale for the legal probe into Kerviel's actions included a message sent by the second trader to Kerviel over the bank's computer system.
Societe Generale announced on January 24 that it had lost 4.82 billion euros ($7.09bn) in cleaning up the unauthorised transactions.
Since the case broke, Societe Generale has faced questions about its future, amid speculation it could be bought out or broken up, and about how Kerviel's activities went unnoticed or ignored.