The 30-year-old has signed a three-year deal with the Italian champions in a move that includes a potential £1.5m in add-ons.
Hernanes joined Inter in January 2014 and scored five goals in 34 appearances last season.
Juventus lie 17th in Serie A having lost their opening two fixtures this season.
Coach Massimiliano Allegri has lost several key players this summer, including Carlos Tevez, Arturo Vidal and Andrea Pirlo.
After the international break they welcome table-toppers Chievo Verona to the Juventus Stadium on September 13.