RSPCA officers said Bradley Powers' English Mastiff Hercules was so emaciated, with "nearly every bone visible", he had to be euthanized.
Powers, 51, of Swansea, admitted causing unnecessary suffering to the dog at Swansea Magistrates' Court.
He was given a 12-week prison sentence, suspended for two years.
RSPCA said inspector Leigh Summers visited Powers' home in November last year to find an "extremely skinny" Hercules with a flea infestation.
Insp Summers said it was one of the "worst cases of neglect I have ever seen as an inspector - you could see nearly every bone in his body".
Insp Summers added: "As well as not treating him for his weight loss, Hercules also had to suffer with a flea infestation, something which is easily preventable."
She added: "Although his owner has now been convicted and has been banned from keeping animals, his actions meant that Hercules lost his life and was not able to get the second chance he deserved."
A man who failed to take his severely-underweight and flea-infested dog to a vet has been banned from keeping animals for 10 years.