Harley, who has signed a two year deal at Vicarage Road, told the club's official website: "I'm chuffed to bits with having signed.
"I can't wait to get down to work with the promotion chase.
"It wasn't so much a decision I had to make, just more the case that Watford wanted me and I wanted them.
"It was as easy as that, I didn't even have to think about it."