The 23-year-old has had one-year loan spells with Lecce, Padova and Grosseto over the past three seasons, and he spoke of his delight at the move, which will see him at the club until June 2017.
"I am happy that the deal has been completed," Donati told the club's official website. "Bayer are a big team in Germany and also have a big reputation in Italy. I am really looking forward to the Bundesliga and it will be a huge challenge to play in the Champions League with Leverkusen.
"I will give my all to help B04 achieve their goals."
Leverkusen general manager Wolfgang Holzhauser also spoke about the move for the youngster, saying he is exactly the kind of player the club are trying to sign.
"With Giulio Donati we have a young, aspiring player who fits perfectly into the profile of Bayer Leverkusen," Holzhauser said. "He was a mainstay of the Italy team at the Under-21 European Championship in Israel, and we are confident he will fit in at right back very well."
Sporting director Rudi Voller also had his say on the purchase of Donati, lauding his potential.
"He is a player with great potential, and is a regular at Under-21 level," Voller said. "He has played lots in Serie B and impressed us a lot in the Under-21 tournament. He will bring us lots of joy in the future."
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