The 15-year-old, who will turn 16 in February, has signed a deal until the summer of 2021.
Dembele made headlines just over two years ago when he appeared as a second-half substitute in a Development League game against Hearts, playing against players seven years his senior.
Since then he’s attracted interest from around Europe, with any club able to tempt him away from Parkhead before his first professional deal had been signed.
However, Dembele, who was born in London before moving to Glasgow with his family while he was still a baby, has decided to stick with the club he joined at the age of 10.
He told the club’s website: “It means a lot to me to start my professional career at Celtic. 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.
“I’d like to say thank you to everyone, and I’d like to thank everyone for everything they’ve done for me. “I’d also like to thank everyone for the training I’ve received, the discipline I’ve been taught, and the advice I’ve been given all the way.
“Getting attention at a young age has been hard, but I’ve obviously had my family to help me through that. You need to be strong mentally, you need to stay grounded, and you just need to get on with everything that you do on a daily basis.
“Seeing players come through the Academy and into the first team gives you hope, because you know they’ve taken the same path. That means you’ve got a chance to make it into the first team if you work hard and give it your all.
“My brother Siriki, who plays for Peterborough United, first taught me how to play football. I wasn’t always interested in football, but he brought me in and he helped me. He helps me in everything I do. If I have a bad game, he’ll talk to me and he’ll say, ‘don’t be down, keep your morale high’.”
He added: “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 per cent. 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.”