The 23-year-old, who scored 11 goals in 22 league appearances last season, has put pen to paper on a four-year contract at Vicarage Road in a deal understood to be worth around £4.6million.
Berghuis becomes Watford's 10th summer signing ahead of the new Barclays Premier League season.
Jose Manuel Jurado, Miguel Britos, Sebastian Prodl, Giedrius Arlauskis, Matej Vydra, Jose Holebas, Etienne Capoue, Valon Behrami and Allan Nyom have also penned permanent deals with the newly-promoted Hornets.
"I'm very excited to be here," Dutchman Berghuis told Hornets Player
"There were long discussions about whether I would come and I'm looking forward to playing here.
"I was happy at my old club but if you want to make steps in your career you have to take other steps and prove yourself again.
"This club wants to play football and I am a footballing player
I like it.
"(The Premier League) is the biggest competition with a lot of good teams
The teams are almost all good, instead of just two or three, and you have to prove yourself every week.
"Watford have a good vision and I know they want to stay in the Premier League."
Source : PA
Source: PA