Credit Agricole SA and Societe Generale were both cut one notch from Aa1 to Aa2 and Aa2 to Aa3 respectively.
The agency said it would also review BNP Paribas for a possible downgrade but for the moment believed the bank's profitability and capital base could support its exposure to Greek, Portuguese and Irish debt.
Christian Noyer, Governor of the Bank of France, was not put off by the downgrade, saying: "Moody's had a higher rating than the other agencies so it's just put them on the same level or slightly better than the others".