The 43-year-old woman was killed in Chao Fa Road West, near the intersection with Kwang Road, as she approached Central Festival Phuket from Chalong Circle about 8.08pm.
A man in a black Mitsubishi four-wheel drive pulled alongside the woman's white Chevrolet.
The killer - probably in a passenger seat - fired one shot through the window, hitting her in the head as she talked on a mobile telephone, killing her instantly.
The killer's vehicle was driven off towards Central, leaving chaos behind as the victim's car veered into the kerb and knocked over a light pole. The victim had a seat belt on.
Traffic was held up and a crowd probably numbering hundreds gathered as first paramedics then police investigators descended on the grisly execution scene.
Forensic crews were still searching for clues tonight.
The vehicle had Phuket plates but the owner was recorded as being from Nonthaburi province, across the river from Bangkok.
The woman, wearing a blue polo top and white jeans, slumped at the wheel. The crowd continued to look on as her body was removed from the vehicle.
It is believed the woman lived in Yaowarat Road, Phuket City, and had a business connection with an energy drink manufacturer.