Gen Curtis M Scaparrotti, commander of US Forces in South Korea, apologised today for the "possible theft" from two databases of private details of South Koreans such as names, contact information and work history.
About 16,000 current and former workers, almost all of them Korean nationals, and people who have sought jobs with the US military in South Korea, are affected by the incident.
The US military said no classified military data was compromised as the databases were on a separate network.
US Forces spokesman Christopher Bush said an investigation by the US Army was underway to determine who was responsible. The US has around 28,500 soldiers in South Korea as a deterrent against the North. The 1950-1953 Korean War ended in a truce so the peninsula is still technically at war.