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.