He then continued ransacking the shop, finally leaving with 106 bahtweight of gold jewellery estimated to be worth more than B2 million.
The customer, Apichet Chuklieng, 32, died later in the nearby Phuket International Hospital of a single bullet wound to his chest.
The robber was caught on CCTV entering the shop wearing black clothing with white stripes from the shoulders and down the arms of his top, a black motorbike helmet and a surgical mask to conceal his face. He is esytimated to be about 165cm tall.
The video shows him, carrying a bag, marching behind the counter and waving it at the staff, all women, who flee. When Mr Apichet approaches, he calmly shoots him and then leans over the counter to take another shot.
It is understood that the gun misfired, but Mr Apichet was in no condition to do any more.
The gunman then continues along behind the counter, stuffing gold into his bag. He then walked out of the shop where he sent security staff diving for cover with a shot in the air, before riding off on a motorbike.
Staff later said the robber told them, “This is a robbery. If you run I will shoot you.”
Police hope that witnesses may be able to give the a description of the motorbike or even its number. Forensics officers are also at the scene looking for clues.
Phuket Police Commander Maj Gen Choti Chavalviwat asked anyone with information about the robbery to contact investigators.
He said he thought the robber might be the same man who robbed the Kasikorn Bank branch in Rawai on January 7 this year, threatening staff with a gun and stealing B260,000. A reward of B100,000 was offered after that theft for information leading to the robber's capture.