South Korean police said they had begun investigating Samsung Card over the alleged leakage of personal information on its customers.
The Seoul Metropolitan Police Agency said in a statement on Thursday that a Samsung Card employee was suspected of leaking the personal details of 800,000 customers.
Police raided the company's Seoul headquarters late Thursday to obtain evidence related to the case.
Local newspaper the Chosun Ilbo quoted a company official as saying that the suspect appeared to have sold the information to illegal spam providers.
The incident follows similar customer data breaches including a hacker attack targeting SK Communications in July and massive network failure at commercial bank Nonghyup in April.
South Korea recently drew up a cyber security master plan after a wave of hacking attacks against global agencies, companies and its own financial firms.
Shares in Samsung Card slid 2.2 percent as of 0014 GMT.
(Reporting by Ju-min Park; Editing by Jonathan Hopfner)