Deloitte said late Monday that hackers had accessed the firm's email platform in what a statement on its website called a "cyber incident."
"Only very few clients were impacted" by the attack, according to the Deloitte statement, and "no disruption has occurred to client businesses, to Deloitte's ability to continue to serve clients or to consumers."
All affected clients had been contacted, Deloitte said.
Deloitte also said it conducted an "intensive and thorough review," including the use of internal and external cybersecurity and confidentiality experts, and contacted governmental authorities once it discovered the hack had occurred. The firm did not elaborate.
The company did not provide other details about the cyberattack, including the time of the attack and number of email accounts that were affected. Efforts to get additional information from a Deloitte spokesman were unsuccessful.
The announcement comes less than a week after the Securities and Exchange Commission on Sept. 20 announced that hackers in 2016 obtained non-public information from its EDGAR corporate filing system that could have been used in illegal trading.