Police were notified of the remains by a call to the national police hotline 191 just after midnight.
When police arrived at the scene, they found a bone protruding out of the edge of the drain, which measured about 1.5m long and 50cm wide and was full of water.
Upon recovering the rest of the remains, officers uncovered a soil-clad corpse dressed in a white T-shirt and long blue trousers. Some dried flesh remained attached to the skeleton.
The remains, in a very advanced state of decomposition, had the skull detached from the body.
Police inspecting the area today found no weapon, or signs of a struggle.
The remains have been sent Vachira Phuket Hospital for medical examination, said Lt Col Udom Petcharat, Deputy Commander of Investigation of Phuket City Police.
“It is currently unclear who is dead,” he said. “Most of the remains are decomposed to bone. Further, there was no evidence found at the scene around the body,
“We are waiting for the autopsy results from the hospital,” Col Udom added.