Officers were called to the scene, a roadside drain along Srisoonthorn Rd near Blue Tree Phuket, at 9am.
Police believe that the baby had been dead at least two days before it was found this morning.
The body was recovered from the drain and taken to Thalang Hospital.
Police believe the newborn girl was dumped in the drain, either at the location where the body was found or nearby.
DNA samples will be taken from the baby’s remains in case they can be used to identify the mother, police said.
Offices are now checking CCTV in the area in the hope of locating the mother. Police are intending to press charges, officers noted.