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."

The UKIP leader tweeted he was "at a standstill for 90 minutes" while heading to Thursday's show in Bradford.
Highways England said the the A1 northbound was closed in Nottinghamshire after a fatal crash.
In 2014, Mr Farage missed a UKIP conference after getting stuck in traffic on the M4, which he blamed on high levels of immigration.
A UKIP spokesman said on Thursday Mr Farage was being driven from his home in Kent to the show, which usually starts filming at about 20:30.
He tweeted at 19:35 GMT: "Now been at a standstill for 90 mins on the A1. #bbcqt doubtful. Damn."
Updates from this story and more from Nottinghamshire
He was replaced on the show by his deputy leader Paul Nuttall, who travelled from Liverpool to face off against Energy Secretary Amber Rudd, Labour's Shabana Mahmood, Lib Dem president Baroness Brinton and political journalist Isabel Oakeshott.
It was the first major televised debate following the announcement of David Cameron's draft EU reform proposals.
Question Time producers tweeted: "We were making plans for @Nigel_Farage - but travel problems mean he'll be replaced by @paulnuttallukip. #bbcqt."
Despite the late line-up change, the show was broadcast at 22:45, starting with a question on Mr Cameron's EU reforms.
Mr Farage blamed his failure to turn up to a reception with supporters in Wales in 2014 on "open-door immigration".
"It took me six hours and 15 minutes to get here - it should have taken three-and-a-half to four," he said at the time.
On the day of the 2010 general election Mr Farage was involved in a plane crash that left him with broken ribs.
Thursday's delays came when the A1 was closed northbound at Coddington following the crash at about 17:30, in which one man was killed.

The 20-year-old, who has been with the Robins for four years, signed his first professional deal in 2015.
The former Hereford loanee missed much of the 2016-17 term with a knee injury.
"I want to make up for the last couple of years and get back playing. I am very grateful to the gaffer and the medical team. I really want to repay them now," Page told the club website.
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.
[NEXT_CONCEPT]
Nigel Farage missed an appearance on BBC Question Time after getting caught in a traffic jam on the A1.
[NEXT_CONCEPT]
Cheltenham Town winger Adam Page has signed a new one-year contract with the League Two club.