Locatelli, who had been the subject of a bid from Arsenal following Euro 2020 according to Sassuolo's CEO, had his Juve medical on Wednesday, with the Bianconeri posting pictures of his arrival on their Twitter page.
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Locatelli scored with his first-ever shot in Serie A to help Milan come from 3-1 down to beat Sassuolo 4-3 and shot to prominence with a stunning goal past Gianluigi Buffon in the Rossoneri's 1-0 victory over Juve in 2016.
He helped Milan win the 2016 Italian Supercoppa before a dip in form saw him lose his first-team place and he moved to Sassuolo.
With the help of manager Roberto De Zerbi, he re-established himself as a top Serie A midfielder and was an important part of the Italy side which won Euro 2020 -- starting the first two matches and scoring twice in a 3-0 victory over Switzerland.