The man, aged 40 to 50 and balding, had his wrists bound and was wrapped in garbage bags.
Doctors at Vachira Phuket Hospital in Phuket City found stab wounds to the man's head, chest and ankles.
The body was discovered abour 6pm in a pond near Bang Thong Village, off Soi Bang Thong, which runs off the main Patong Hill Road.
Residents investigated a smell, and found the man's body, wrapped in plastic.
Rescue workers brought up the body.
Police are convinced the man was murdered and are now trying to identify him.
Lieutentant Sakchai Chonyong from Tungthong Police Station is heading the investigating team.
The man was about 180 centimetres tall, thickset, balding. He had three stab wounds to the head, five to his chest and two in an ankle.
He was wearing only black underwear. There were no obvious identification marks or tattoos.
After an estimated week in the water, his facial characteristics are no longer obvious.
The Patong Hill road is officially known as Phrabaramee Road. Soi Bang Thong runs off to the left, as vehicles go up the hill from the Phuket City-Kathu side.
It's the first road past the Tungthong police checkpoint.