"I am looking forward to the challenge and that is the reason why I have come," the striker told the Hammers official website.
"I spoke to a lot of very good managers who put a lot of time into signing me; I appreciate that, but in the end I decided West Ham was the club for me.
"I know Glenn Roeder from when I was at Watford and it is great to be hooking up with him and working with him again.
"West Ham is a fantastic football club with a great tradition. I would say for me it is a great move and it is the type of club where every footballer would love to play his football at."