"My goal is to play in the Premiership and I believe I will be doing so soon in the colours of Watford," King told the Hornets official website.
"The manager (Adrian Boothroyd) has had a tremendous influence on me. He is a one-off and the best manager I have ever worked with.
"The fact that he has been prepared to sign for the club for a long time meant an awful lot to me.
"I want to work with him because I know I can become an even better player under his guidance."