The baby was found at around 9pm behind the emergency building near an outpatient pharmacy room, Takuapa hospital director Dr Phaisal Worasathit said.
A plastic bag that contained among other things Bt2,000 in cash and a milk bottle were found beside the baby.
The baby is believed to be about three days old.
Phaisal said the baby weighed about 2.6kg when he was found.