The Serie A side also confirmed that the fee for Ronaldo would be a maximum €23 million, payable over five years. Ronaldo will be 41 by the time the final installment from Manchester United is due.
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Ronaldo, 36, will return to Old Trafford after leaving them in 2009 for a then world record £80m to Real Madrid. He moved to Juventus for £100m in 2018, where he won the Serie A title twice. In total, Ronaldo scored 101 goals in 133 appearances for Juventus.
The Serie A side penned an emotional tribute to the Portugal international on their website, which read: "On 10 July 2018, two icons of the European and football world came together -- Cristiano Ronaldo became a Juventus player. Today, after three years together and 133 appearances, 101 goals scored and five trophies won, that chapter has come to an end. ... The paths of CR7 and Juventus have gone their separate ways.
"On that day in July, when Cristiano arrived in Turin, the electric feeling of those great days could be felt through the air. The Juventus fans welcomed CR7 like a king, eager to see him on the pitch and cheer with him. It was a great story.
"The unforgettable moments experienced together were many. We could recite them and relive them, but it would not help, because everyone has their own special memory linked to CR7. Everyone has a shared emotion that does not need to be dusted off, because everyone will always remember what they felt in walking this stretch of road together.
"Today that bond born on July 10, three years ago, has been dissolved, but what has been written will forever remain. It has been an incredible journey."
Ronaldo spent six years at United from 2003 to 2009 in his first spell there, which included 292 appearances, 118 goals, three Premier League titles and a Champions League triumph in 2008.
Sources told ESPN he had his medical ahead of his return over the weekend, with official confirmation from United due on transfer deadline day. The transfer window closes on Tuesday at 11 p.m. BST (6 p.m. ET) and Ronaldo could make his second debut for the club on Sept. 11, at home to Newcastle in the Premier League.