A 63-year-old man died following a car crash at the U-turn along the 84.1km mark of the Lumut Highway on Thursday night.
The driver of the other vehicle, meanwhile, sustained serious injuries.
Four other passengers who sustained minor injuries are reported to be in stable condition.
The accident was reported on the emergency hotline 993, the Royal Brunei Police Force (RBPF) stated.
Preliminary investigations revealed one driver was found unconscious when RBPF personnel arrived at the scene.
The drivers and the other passengers were rushed to the Suri Seri Begawan Hospital, Kuala Belait by Emergency Medical Ambulance Services (EMAS) personnel with help from the Fire and Rescue Department.
The RBPF is investigating the cause of the accident, and urged drivers to always be alert and drive carefully.
They also highlighted the importance on the use of safety belts, adhering to speed limits, and to pay attention on the road, especially at night.