The temperature in Phuket was high, ranging from 38º to 41º Celsius.
A Russian, Igor Kiselev, 59, was sent to hospital for treatment after he passed out at Karon beach.
Medical staff tried for an hour to revive him before he was pronounced dead.
An examination showed no sign of injuries and the doctor suspected that he died from a heart attack brought on by the hot and humid weather.
His death was reported to Kathu police station about 7pm by the hospital's emergency unit. The Russian embassy was contacted, according to Thairath newspaper.
The hospital also informed the police that two other foreigners were admitted to the hospital after falling unconscious.
Walter Federick Bell, 62 (nationality unknown) passed out on the beach road in Patong and later died at the hospital, and a 45-year-old Burmese man, unnamed, was taken from Karon to the hospital after fainting. He, too, was later pronounced dead.
The doctors said it was possible the three men died from heatstroke combined with the consumption of alcohol, which would also have caused an increase in their temperatures.
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