The Lyon striker has joined Atletico on-loan to June with a permanent option worth €33.5m included.
Dembele said, "It is a great challenge for me. This is a great club known throughout the world. It is a great step for me. I am delighted and can't wait to start.
"When you arrive in a new country and you don't speak the language, it is always difficult at first. I will do my best to learn Spanish quickly.
"I hope you can help me to get to know my teammates and the club in depth."
He continued: "It's a big club with a big stadium. I hope the fans can come back to cheer us on soon. I could have talked to (Thomas) Lemar, but I didn't want to before signing the contract. Now that I have signed, I will talk to him to tell me about the club and colleagues.
"I hope to get along with everyone."
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