Almost 1,000 North Korean defectors have had their personal data leaked after a computer at a South Korean resettlement centre was hacked, the unification ministry said.
A personal computer at the state-run centre was found to have been "infected with a malicious code".
The ministry said this is thought to be the first large-scale information leak involving North Korean defectors.
The hackers' identity and the origin of the cyber-attack is not yet confirmed.
The North Korean government does not know the identities of all citizens who have defected.
Some may be considered a "missing persons" case, or they may have even been registered as dead.
Some 997 North Korean defectors have now been informed that their names, birth dates and addresses have been leaked but it is not clear what impact this will have.
Analysts say there are some concerns that the leak could endanger the families of the defectors that still remain in North Korea.
Sokeel Park, South Korea Country Director for Liberty in North Korea, an international NGO that assists North Korean defectors, says this hack will make other defectors feel less safe living in South Korea.
They may change their names, phone numbers and home addresses.
On 19 December, the unification ministry became aware of the leak after they found a malicious program installed on a desktop at a resettlement or Hana centre in North Gyeongsang province.
These are the institutes which the South Korean government run to help the thousands of defectors who have come from North Korea adjust to life in the South.
The ministry said that no computers at other Hana centres across the country had been hacked.
One expert on North Korean cyber-warfare, Simon Choi, believes that this might not be the first time a Hana centre has been hacked.
"[There is a North Korean hacking] group [that] mainly targets [the] North Korean defector community... we are aware that [this group] tried to hack a Hana centre last year," he said.
However, he added that it was not yet clear if any North Korean groups were responsible for the latest attack.
Investigations by the ministry and the police are currently ongoing, with the ministry saying it would "do its best to prevent such an incident from happening again".