The Belgian completed his £15million switch from Fulham last night and will link up again with his close friend and fellow countryman.
Dembele said: "I know Jan from when I was 10. We played together then.
"He's a great guy and he's said a lot of good things about the club.
"He says there are friendly people here and, for me, that's very important - to feel great somewhere."
The midfielder, 25, signed a five-year deal with the North Londoners, despite reports of some late interest from Manchester United.
He added: "I'm happy to join Spurs, it's a fantastic club.
"You can see over the last few years they have played well. Everybody knows it's a good team and a good club and that's why I wanted to come here.
"They have plans to do lots of good things and I want to be a part of it."