"I am delighted to have tied up the formalities with Adrian (Boothroyd) and the Watford board," said King.
"The manager has had a tremendous influence on me and is the best manager I have ever worked with.
"Under his guidance, I know I can become an even better player
"My overall goal is to play in the Premier League and I believe I will be doing so in the very near future wearing the colours of Watford."