The 29-year-old will provide competition for number one Djordje Petrovic and step in should the Serb lose form or suffer an injury.
The Cherries will not pay a loan fee but will cover the Italian's full salary. They are also due to pay a £1m penalty fee if they do not trigger the option to buy him for £12m, plus a further £2.5m in add-ons.
Gregorio is an Inter Milan youth product who has spent his entire career in Italy, going from the third division on loan at Renate to Serie A with Monza in 2020, before joining Juventus in 2024.
The move is significant as it signals that Marco Rose's side have largely completed their transfer business this summer, barring any surprises.
They have signed striker Alvaro Rodriguez from Elche, defender Antonio Silva from Benfica and full-back Juanlu Sanchez from Sevilla on permanent deals.
However, keeping hold of key players such as midfielder Alex Scott, who was the subject of a £64m bid from , and forward Eli Junior Kroupi has also been important.
Bournemouth wanted to keep their strongest possible squad together as they prepare for their first European campaign in the Europa League.
