Budget Minister Francois Baroin said hackers had mounted a "spectacular" attack on the ministry's IT system in December.
"What was targeted was the organization of the G20," he told Europe 1 radio. The dossiers of individual French taxpayers had not been targeted, he assured.
Although it was not yet clear whether files had been copied, Baroin said it was very likely the hackers had stolen information.
French authorities had unconfirmed "leads" on the origin of the attacks, which "probably" came from "outside", he said.
Paris Match magazine, which broke the story on its website Monday, said the hackers tapped into 150 computers between December and last week, and copied several documents relating to the G20.
The ministry took 12,000 computers offline at the weekend in an effort to secure the system.
The G20 is made up of the world's leading developed and emerging economies.
Last month, G20 finance ministers held talks in France to discuss global economic imbalances.