Pol Capt Passawat Patphan, deputy inspector of the Bang Pakong police station, said the accident happened at 3.30am at kilometre marker 37 of the inbound Bangna Trat highway.
Supassorn Saeng-aroon, 17, and her pillion passenger, Natthida Malaithong, 17, were taken to Bang Pakong Hospital.
Supassorn said a 18-wheel truck grazed her motorcycle, causing it to hit the roadside barrier and her friend was thrown off the motorcycle and hit a pillar. She was severely injured and unconscious.