The 23-year-old has signed a four-year deal at the BayArena and moves for an undisclosed fee.
Donati has represented his country at Under-20 and Under-21 level and, after spending the past two seasons on loan at Serie B clubs, is hoping to step up and make an impact in the Bundesliga.
"Bayer is a top German club and also has a good reputation in Italy," he told www.bayer04.de.
"I am looking forward to the Bundesliga and being in the Champions League this season with Leverkusen is a very special challenge.
"I'll give everything to achieve our goals with Bayer."
Donati played every minute of Italy's recent run to the European Under-21 Championship final in Israel and clearly caught the eye of Leverkusen director Wolfgang Wooden.
"With Giulio Donati we have a young, aspiring and committed player, which fits perfectly into the profile of Bayer Leverkusen," he said.
"He was a mainstay of the successful Italian team at the Under-21 European Championships in Israel.
"We are convinced that Donati will successfully fill the position of right-back."