Tony Kenway, who was originally from Southampton, was shot dead moments after leaving the the Sanit Sports Club in Bang Lamung district, in Pattaya.
Local media say witnesses have described the shooter to police as a large man who was wearing a white t-shirt and a black jacket.
CCTV footage shows Mr Kenway, who moved to Thailand several years ago, walking towards his Porsche Cayenne GTS before he was targeted at around 10.20am.
As he opened his door and sat in the car, the 39-year-old was followed by his killer before he was shot at the wheel of his vehicle.
The father was followed by the gunman before he was shot at the wheel of his car
Next the gunman can be seen fleeing the scene as he made away on a motorbike after shooting the father of four.
Despite the efforts of the emergency services, Mr Kenway was pronounced dead at the scene.
A spokesperson for Nong Prue Police in Pattaya said: “We have started an investigation and we are looking for the attackers.
“There was a motorbike waiting and the killer escaped with another person riding it. There are two people we are looking for.
The gunman can be seen fleeing the scene on a motorbike after the shooting
“We believe there was a conflict of interest in a business that Mr Kenway had been involved in. We know nothing more than that.”
Mr Kenway, who ran a website design business, had reportedly been at the gym before heading back to work.
A spokesperson from the Foreign and Commonwealth Office, said: “We are in contact with the family of a British man following his death in Pattaya, Thailand.
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“Our sympathies are with them at this difficult time. We are in touch with the local police.”