A passing motorcyclist was killed by a gunshot from an angry pickup driver who fired at the driver of a six-wheel truck in Samut Prakan district on Monday night.

Motorcyclist Supakorn Chankhati, 24, was shot in the head while taking his girlfriend to a hospital, about 7pm on Bang Phli-Tamru Road in tambon Bang Poo Mai.

Rescue workers rushed him to nearby hospital, where he died.

CCTV footage led police to arrest Burapa Jiamyu, 26, on Monday. The man allegedly admitted to the shooting.

Mr Burapa said he had been driving a pickup truck used for ice deliveries and had been driven into a rage by a six-wheel truck driver on Bang Phli-Tamru Road. They had argued at a petrol station. The truck driver had been holding a steel bar but drove off when Mr Burapa produced a gun.

Police said the pickup driver admitted he had then overtaken and stopped ahead of the six-wheeler, and fired a gunshot.

Mr Burapa said he intended to fire only a threatening shot, but the bullet had hit a passing motorcyclist. He had earlier stolen the gun from a teenage neighbour, and dropped it after the shooting.

The victim's 23-year-old girlfriend said she had a stomachache and Supakorn was taking her to a hospital on his motorbike. She heard a gunshot and Supakorn collapsed in front of her. Fortunately, she was able to reach around him and control and stop the motorcycle.

She said she saw the accused gunman at the scene.