The 24-year-old's long association with Nottingham Forest ended last week upon the expiry of his contract.
"Lewis is a very talented player and an excellent signing for this football club," Watford boss Gianfranco Zola told the club website.
"He will give strength to our squad and I'm sure our supporters will be excited to see what he can do in a Watford shirt."
Source: Forest MAD
Source: FOOTYMAD
