De Gea came close to joining Madrid in the summer of 2015, but a faulty max machine saw the Spaniard's expected move break down at the final moment.
The 27-year-old has developed into arguably the best goalkeeper in the world over the last few seasons, and speculation linking Real Madrid with a move for the former Atletico Madrid stopper has not quietened.
De Gea's current deal with United will expire in the summer of 2019, and Bellamy is certain that the Spain international will ultimately end up at the Bernabeu.
"He is the real deal and I have always felt that Real Madrid have been looking for him to be their number one for a number of years," Bellamy told Sky Sports News. "And it is just a matter of time before they get him.
"His footwork is brilliant, he anticipates as well, he can read forwards and if you watch, he makes a lot of his saves with his feet. He is incredible. He asks the forward so many questions when you are gunning down on him and he is a real top goalkeeper."
De Gea has kept 18 clean sheets in 31 appearances for United this season.