The deal will see Ronaldo earn £106m over the four years on his current contract.
The 33-year-old has had his first training session with new Juventus teammates today in Turin.
And Napoli Presi De Laurentiis has mocked the deal.
“Spending so much money to buy only one player at the end of his career is very dangerous,” De Laurentiis told La Vertia.
“Let’s remember that Cristiano Ronaldo is of a certain age and in terms of the club’s balancing of accounts his signing is a risky move.
“The signing of Ronaldo is out of any standard.
“If you start to pay such absurd and out-of-the-ordinary salaries, sooner or later this turns against you.”
By signing Ronaldo, Juventus became the first Italian club to break the £100m barrier for incoming transfer.
They also smashed the previous transfer record in Italy by landing Gonzalo Higuain from Napoli in a £75m deal two years ago.
“(Ronaldo) has at his side what – perhaps- is the most intelligent of sports agents: Jorge Mendes,” De Laurentiis added.
“I had been with him for breakfast in Portugal and in those days we talked about the goalkeeper Rui Patricio.
“Then, when it was clear that Real wanted to left home (Real Madrid), he called me and said: ‘Listen Aurelio, can we play this game?’
“Was it too expensive? Nah. That said, it’s a nonsense. I imagined a model in which the costs were matched exclusively with the proceeds mirrored to the relative costs of the operation.
“I had told Mendes: ‘Do you think that Ronaldo will make me rich? If so, we do it so the first €250m turnover goes to Napoli, and the next €100m are due to Ronaldo’.
“If he could actually earn (those figures), he would have nothing to risk or to fear.
“But then Juve arrived, and – behind FCA (Fiat Chrysler Automobiles, who have part-funded the Ronaldo deal) – raised the stakes.”