He joined Tottenham from MK Dons in February 2015 and went on to score 10 goals to help Tottenham to a highest-ever third in the 2015-16 English Premier League.
The 20-year-old was then selected for the summer's European Championship and started three of the four matches in France.
"I'm happy to be at this club and I can't thank the manager and all the staff enough for everything they've done for me, and the fans, they've been right behind me since I signed. Hopefully there are a lot of good times to come," Alli told Tottenham's website.
He made his debut just over a year ago against Sunderland, and added: "I remember that game, I didn't expect to play as quickly as I did and make an impact in the team.
"It's happened so quickly, starting at Tottenham, playing for England ... it's been a crazy year for me and there are so many people I have to thank."
This season he has played in all five of Tottenham's EPL matches and scored once, in a 4-0 win at Stoke earlier this month.
"I have to keep trying to improve. I can't get carried away, I have to keep trying to make each year better than the last. This will be a tough year to top but hopefully I can do it," he said.
"I've tried to grow up. I've got great people around me to help keep my feet on the ground and I'll keep working hard on my football and keep trying to improve."
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