Police in Pattaya, an eastern Thai city renowned for its long beach and seedy nightlife, said Tony Kenway, 39, was shot around 9 am (0200 GMT) after getting into his red Porsche.
"The shooting happened just after 9 am when a British man left a sports club and got into his car," Police Major General Somprasong Yentaum, head of Chonburi police, told AFP.
"He was shot once in his head, the bullet casing was a .38 calibre," he added.
It showed a man in dark clothing walk up to the victim's car, open the door and fire a shot before jumping on a parked moped and making his getaway.
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Somprasong said there was a crowd outside the gym that witnessed the killing and that the man's wife was baffled as to why he may have been targeted.
A spokesman at the British embassy in Bangkok confirmed a national had been killed and that officials were in touch with both the man's family and local police.
Pattaya boasts a large expat population and is notorious for its go-go bars and links with organised crime.
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