The 25-year-old Argentina international has joined the Hornets on a five-year deal, the club announced.
His arrival gave the Premier League outfit a second new player of the day after they earlier finalised a move for defender Younes Kaboul from Sunderland.
The 30-year-old centre-back penned a three-year deal with Walter Mazzarri's side.
Pereyra, who has 10 caps for Argentina, scored six goals in 68 games after joining Juventus in 2014.
He has helped the Italian giants to two Serie A titles and two Coppa Italia successes while he was a second-half substitute in the Champions League final defeat to Barcelona in 2015.