The 25-year-old has signed a five-year contract at Vicarage Road, becoming the club's second signing of Friday following the acquisition of defender Younes Kaboul.
Watford has signed Roberto Pereyra from Juventus after agreeing a five-year contract with the midfielder.
Walter Mazzarri's side has long been linked with a move for the Argentina international and finally completed the capture of the 25-year-old on Friday.
Pereyra, who was Watford's second signing of the day following the acquisition of Younes Kaboul from Sunderland, must wait for a work permit to be granted before he is available for selection.
OFFICIAL: #watfordfc are delighted to confirm the signing of Roberto Pereyra from Juventus on a long-term deal.