The 21-year-old has signed on an 18-month loan deal which has the option to become permanent for a reported £56million.
Fernandes becomes the club’s first signing of the Jose Mourinho era and will provide vital cover in the middle of the park following a three-month lay-off for Moussa Sissoko, who has had knee surgery.
He will wear the No 30 shirt in north London.
“I’m very happy to be here today – I make my dream a reality,” he said. “For me, it’s one big dream to come to this big Club. I will try to give my best for the Club every day, in every training session and in every game and I will try to help the team wherever I can, working hard every day for this shirt.
“I have no words to say about the stadium. I’ve never seen anything like this – it’s unbelievable!
Fernandes could replace Christian Eriksen with the Dane appearing to be edging closer to the exit door.
A move to Inter Milan is in the offing with the two clubs understood to be negotiating over a fee.
Mourinho says that Eriksen should be proud of his stint in north London.
“He played very well. Played very well, very professional which is what I expect from him,” Mourinho said of his display against Boro.
“If his decision is to leave I think he has to leave with his head up, if he gives everything which is what he tries to do for the team.
“Fans, it’s always to respect, we have to respect but I think the boy did it for us today.”