A car with four people inside crashed into a US armoured vehicle in Pocheon, north of Seoul.
Four South Koreans died and a US soldier suffered injuries in the car accident, according to Yonhap news agency.
According to reports, the US armoured vehicle was heading to a military base near Seoul, when the passenger car collided with the rear of the US military vehicle.
Victims of the accident were taken to hospital and received emergency treatment including CPR, but succumbed to their injuries later.
Some 28,000 US troops are stationed in South Korea to deter an attack by North Korea.