The smell led residents to call police about 3.30pm.
Police found the front door of the second-floor condo locked, but on investigation found the sliding door on the balcony ajar.
The odor was emanating from the condo through the opening.
Having gained access, the officers found the body of a man lying face down on the floor, not far from the door.
He was bleeding from the nose and mouth. The condo, police said, appeared to have been ransacked.
The man was about 170-180 centimetres tall and weighed about 80 kilos, according to paramedics who attended the scene.
Neighbors told police that the condo was owned by a 68-year-old British man who returned to his home country in March.
They said another expat, whom they knew only as Sam, dropped by to take care of the flat but did not stay there.
Sam was last seen about four days ago, near the condo block. Police have not named the location of the condo.
The body was taken to Patong Hospital.
Officers from Patong's Kathu Police Station are continuing their inquiries and awaiting the results of a medical examination of the body.