The England international brought gifts including the shirt he was wearing when he scored his first Premier League goal – and then stayed for two hours playing computer games and chatting.
Tony Clark, a big Tottenham fan, was diagnosed with the rare disease in October and is now undergoing chemotherapy.
His distraught parents began an ambitious bid to get Tony’s hero Alli to visit him in Great Ormond Street Hospital in Bloomsbury, central London.
They made contact with the 20-year-old’s mum through Facebook and were told the midfielder would try to visit the day after Spurs’ Champions League clash against Bayern Leverkusen.
And to Tony’s disbelief, Alli indeed arrived 48 hours later.
Tony’s dad Anthony said: “I thought coming back from Germany in the early hours of the morning and still having to train the next day would mean he wouldn’t make it.
“So we couldn’t believe it when he turned up with shirts for Tony and then stayed for two hours.
“They played FIFA and Tony was Tottenham so he could play as Alli, and Dele played as Barcelona."
What's hot
He added: “It’s incredible that he found the time to come just being 20 himself, and the shirt he brought must have had real sentimental value for him so it just means so much to us.
“I don’t know if Dele Alli knows just how much joy and happiness he gave my son in the biggest battle he will probably ever face in life.”
Anthony’s family has already raised £3,000 to fund Tony’s transport and care costs.
The youngster, who plays football for his local youth side in Kentish Town, north west London, has an especially rare cancer, which is difficult to treat.
Doctors expect he’ll need regular treatment for around three years.
Anthony’s family comes from generations of Tottenham fans.
All, who used to play for Milton Keynes Dons, has scored three times in nine Premiership games so far this season.
He played the full 90 minutes in the 0-0 draw in Leverkusen, west Germany, the night before the hospital visit. The ace is expected to start for Spurs in their crucial North London Derby against Arsenal this weekend.