"Darius fits the new culture here - he's young, hungry and part of the new breed of footballer that I want to bring to this club," manager Adrian Boothroyd told the clubs official website.
"He's been one of my top targets during the summer and when you see him in action you'll see why - he's big, powerful, aggressive, mobile and gets goals.
"I'm really pleased he's joined us and he now has a chance to grow into a top striker at Watford."