A housemaid who called at the villa in the eastern Cape Panwa region to clean the premises reported finding the body to Phuket police about 8.45am today.
Initial reports say the man, aged 69, was found face-down dressed in black boardshorts. No cause of death has been established.
The man was last seen alive on Saturday. His passport has yet to be found.
The villa is in Soi Santi Suk, which runs off Sakdidet Road towards Nam Bo Cape.
Police Lieutenant Anurak Klangnarong, of Vichit Police Station, south of Phuket City, said the man's Thai wife had been informed of his death and was heading home from Korat, where she had been visiting family members.
the body was taken to Vachira Phuket Hospital in Phuket City where a physical examination would be made to determine the cause of death.
Police say the man may have missed taking daily medication because his wife wasn't there to care for him. It was possible he fainted and fell into the pool, police said.
Cigarettes were found in an ashtray by the pool and a beer glass was in the pool. Sunglasses were floating nearby.
Phuketwan has been supplied with the man's name but is withholding publication until relatives in the US have been notified.