Finance Minister Elena Salgado says the owners of the accounts have been notified and asked to clarify whether they have declared the money in the accounts.
Daily El Pais says the accounts are in the Swiss branch of the bank HSBC and that French authorities tipped Spain off to their existence.
Salgado declined Thursday to give details but said the probe was part of Spain's campaign to combat tax fraud.
HSBC spokesman Pascal Dubey said they had not received any official confirmation of the probe from Spain.
