The highly-rated midfielder joined Celtic aged 10 and made headlines when he made his under-20 debut against Hearts in October 2016.
The Scottish club announced on Monday Dembele signed a deal to stay at Celtic Park until at least 2021.
"It means a lot to me to start my professional career at Celtic," he told the club website.
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"I came here at a young age and, after all the stuff me and family have been through together, it'll be good to make them proud.
"My goal is to play for the first team. As soon as I can, I'm going to try to get into the first team, 100 percent. In the reserves, every training session, every game – it's all to prove myself to the manager that I can play in the first team."
The London-born youngster has featured for Scotland under-16s and England under-15s on the international stage.
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