The company, a major lender in the country, said the hacker demanded a payoff in return for not releasing the information, Yonhap News Agency reported.
Hyundai said critical financial information was not released. Investigators were unsure if the hacker obtained cellphone numbers and e-mail addresses for the 420,000 people whose data was compromised.
On Friday, Hyundai's 2 million customers were asked to change their passwords.
The Financial Supervisory Service said an investigation into the leak will begin Monday.
Hyundai provides personal and car loans and home mortgages.
Celebrity Families of 2014 [PHOTOS]