The 17-year-old was found unconscious in his dorm in Leicestershire on Sunday and taken to hospital, where he died on Tuesday, the BBC has been told.
He had been enrolled in a football academy in the UK since late last year.
He was captain of the Thai boys' football team, which was trapped deep inside a cave for over two weeks while exploring in Chiang Rai province.
His grinning face, caught by the torch light of a diver after the boys were found in the cave, was one of the most memorable images from the rescue.
It is not known how the teenager died, but Leicestershire Police said his death is not being treated as suspicious. Reports in Thailand said he suffered a head injury.
Promthep was driven by ambulance to Kettering General Hospital on Sunday afternoon, a spokesperson for East Midlands Ambulance Service said. An air ambulance was also sent to the scene.
In August last year, his team mates rejoiced when Promthep, who they call Dom, announced on Instagram that he had won a scholarship to join the Brooke House College Football Academy in Market Harborough.
"Today my dream has come true," he wrote.
Just six months on, they are mourning the loss of their friend.
In a tweet, British Ambassador to Thailand Mark Gooding passed on "his condolences to all his friends and family."
Promthep studied in Vachiralai Bee School in Chiang Mai before he went to the UK. A diehard football fan, he had been a member of a youth team in Chiang Mai.
His Instagram account is filled with posts on the sport, often accompanied with the hashtag #footballismylife.
One of his last posts in January shows a sketch of his "dream team's football kit" - jersey, shorts, socks and shoes with blue and pink stripes.
After football practice on 23 June 2018, the Wild Boars (Moo Pa in Thai) football team - of which Promthep was captain - raced to the Tham Luang cave on their bicycles. It was one of the team's favourite haunts.
But a sudden storm caused the narrow passageways in the cave system to flood, trapping the boys and their coach inside.
They spent nine days in darkness and without food - while a desperate search effort involving some 10,000 people went on - before they were found by divers.
Promthep turned 13 while he was trapped in the cave. His teammates were aged between 11 and 16 at the time, while their coach Ekkaphon Kanthawong was 25.
The boys used rocks to dig holes to escape, while their coach taught them meditation techniques to help them stay calm and use as little air as possible.
Divers sent them food and letters from their family even as they planned the rescue. They were eventually brought out after being sedated with the drug ketamine.
The rescue made headlines around the world, and various films and books were later made to retell the extraordinary story, including a six-episode miniseries that Netflix released last year.