The Italian side have confirmed the deal via their official website.
Hoffer featured heavily for Die Adler in the 2011-12 season, having been on loan at Kaiserslautern the season before, after joining Napoli from Rapid Vienna in 2009.
The Austrian international, who has been capped 28 times for his country, has two years left on his contract with the Partenopei, despite making only eight appearances, and amassing only 96 minutes in Serie A.
The 25-year-old scored nine times in 30 appearances for the 2. Bundesliga side last season, as they finished 2nd and were promoted to the Bundesliga.
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