The 30-year-old was an attacking target for both mentioned sides during the summer window, but it was known that a huge offer had to be made to lure him away from the Bernabeu.
It was even believed that PSG tabled a deadline day £87million offer for his services, but failed in their efforts as they were eventually rejected.
Real president Florentino Perez revealed the clause via radio station Cadena COPE, as the two interested clubs are likely to be put off by the staggering fee.
“If PSG want Cristiano Ronaldo it’s simple, they have to pay the €1billion release clause,” he said.
“No one is a prophet in football, but we don’t want to sell him in 2016, he still has three years left on his contract.”
The la Liga giants are standing firm on their efforts to keep hold of the Portugal international who signed from United back in 2009 for £80million.
He has been an integral part of the squad since his arrival, featuring 54 times in all competitions last term, scoring a total of 61 goals while providing a further 22 assists.