The Hornets have paid a fee of around £10.4m which could rise to £13m for the Argentinian midfielder.
Pereyra featured in the Champions League final and played 52 times for Juventus two seasons ago, but found first-team opportunities limited last season, making just 16 appearances.
Meanwhile, Watford are also close to signing Sunderland defender Younes Kaboul.
The player joined the Black Cats from Tottenham just one season ago, but wants to leave the club due to personal reasons.