Many major firms in the South have seen customers' data leaked in recent years, either by hacking attacks or their own employees.
In the latest case, an employee from personal credit ratings firm Korea Credit Bureau (KCB) has been arrested and accused of stealing the data from customers of three credit card firms while working for them as a temporary consultant.
Seoul's financial regulators on Sunday confirmed the total number of affected users as at least 20 million, in a country of 50 million.
The stolen data includes the customers' names, social security numbers, phone numbers, credit card numbers and expiration dates, the Financial Supervisory Service (FSS) said in a statement.
The employee later sold the data to phone marketing companies, whose managers were also arrested earlier this month, prosecutors and the FSS said.
The information was taken from the internal servers of KB Kookmin Card, Lotte Card and NH Nonghyup Card.
Regulators have launched investigations into security measures at the affected firms, the FSS said.
"The credit card firms will cover any financial losses caused to their customers due to the latest accident," it said.
An employee of Citibank Korea was arrested last month for stealing the personal data of 34,000 customers.
In 2012, two South Korean hackers were arrested for data from 8.7 million customers at the nation's second-biggest mobile operator.
In November 2011, Seoul's top games developer Nexon also saw the personal information of 13 million users of its popular online game MapleStory stolen by hackers.
In July the same year, personal data from 35 million users of Cyworld -- the South's social networking site -- was also stolen by hackers.