The 18-year-old's current contract runs out in 12 months but he has chose to move elsewhere after not accepting the reported five million euros ($5.58 million) per season contract offered by AC Milan, the club announced on Thursday.
Donnarumma signed for Milan's youth team in 2013 before making his senior debut in 2015 at just 16 years old.
Ronaldo, meanwhile, is reportedly keen to leave Madrid amid scrutiny in Spain over his tax affairs.
Juventus general manager Beppe Marotta has said they "have a duty" to consider signing a player as talented as Gianluigi Donnarumma if he is available, and confirmed they are close to signing Douglas Costa from Bayern Munich.
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"For the moment, nothing will happen".
It is expected that Donnarumma will succeed Buffon as the goalkeeper for Italy, and Marotta also hopes to make him Buffon's heir at Juve.
"The owners do not want to accept any offers".
"It's not true that on Thursday there was tension with Raiola and we argued with him". We were willing to pay him €25m net over five years, which would have cost the club an estimated €50m. We all took the situation in and went our separate ways.
Raiola and Donnarumma have caused nothing short of a storm among Milan supporters in recent weeks due to the delicate nature of the renenwal.