Michael Faherty, 76, was found lying face down near an open fire in his living room in Galway, Ireland. Apart from his body, investigators could find no other damage in the house.
Dr Kieran McLoughlin, the West Galway coroner, said it was the first time in his 25 year career that he had returned a spontaneous combustion verdict, which is believed to be the first in Ireland.
The inquest on Thursday heard how investigators were baffled as to the cause of death, the Irish Independent reported.
Forensic experts found that a fire in the fireplace of the sitting room where the man's badly burned body was found had not been the cause of the blaze.
The court was told that no trace of an accelerant had been found and there had been nothing to suggest foul play. The coroner said he was satisfied nobody had entered or left the house that night.
The fire which killed Mr Faherty was confined to the sitting room. The only damage was to the body, which was totally burned, the ceiling above him and the floor underneath him.
Pathologist Professor Grace Callagy noted in her post-mortem findings that Mr Faherty had suffered from Type 2 diabetes and hypertension but she concluded he had not died from heart failure.
Dr McLoughlin said he had consulted medical textbooks and carried out other research in an attempt to find an explanation.
He said he referred to a book written by forensic pathologist on spontaneous combustion and noted that such reported cases were almost always near an open fireplace or chimney.
"This fire was thoroughly investigated and I'm left with the conclusion that this fits into the category of spontaneous human combustion, for which there is no adequate explanation," he said.