No financial details were disclosed but media reports said Juve received a fee of 12.9 million pounds ($16.86 million) for Pereyra.
The 25-year-old, who featured in the 2015 Champions League final defeat by Barcelona, scored six goals in 68 matches for Juve and helped them secure back-to-back Serie A and Italian Cup trophies in the last two seasons.
Sunderland announced on Thursday that they had accepted a bid from Watford after Kaboul expressed a desire to leave, citing personal reasons.
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The 30-year-old, who left Tottenham Hotspur a year ago, made 23 league appearances for the north-east club last season and helped them avoid relegation by finishing fourth from bottom.
"I'm very happy ... to be a part of Watford Football Club and I'm looking forward to the season," Kaboul said after completing the move.
"I can see it's a family club, there are some good players here and I'm very confident."
Watford, who began the new league campaign with a 1-1 draw at Southampton last weekend, host Chelsea on Saturday.
(Reporting by Nivedita Shankar in Bengaluru; Editing by Tony Jimenez)
