The Belgian playmaker said that he decided to leave Fulham for White Hart Lane because of the club's ambition to move forward.
He told the club's official website: "I'm happy to join Spurs, it's a fantastic club.
"You can see over the last few years that Spurs have played well, everybody knows it's a good team and a good club and that's why I wanted to come here.
"The club has plans to do lots of good things and that's why I want to be a part of it."
Dembele also revealed that he cannot wait to link up with international teammate and long-time friend Jan Vertonghen.
He said: "I know Jan from when I was 10 years old, we played together then. I know him well, he's a great guy and he's said a lot of good things about this club.
"He says there are friendly people here and for me, that's very important, to feel great somewhere. I'm sure I will here."
Dembele goes straight into contention to make his Spurs debut against Norwich City on Saturday.