The former Fulham striker has signed a deal until the end of the 2013/14 season.
"I'm very happy, excited and looking forward to training with the team and to the first game," Petric told whufc.com. "I know [the Premier League] and I'm happy to be back in the Premier League, because it's a fantastic league and I'm really looking forward to being part of this team and this Club.
"There are a lot of good players here and I'm happy to be part of such a good team and I hope we can all celebrate my first goal for West Ham as soon as possible.
"I didn't speak to Sam [Allardyce] until today but I've heard a lot about him and I spoke to Guy Demel, who I know from my time at Hamburg and I'm sure I'll have a good time here.
"I remember playing at West Ham last year and it was a very tough game for us at Fulham. I think we lost 3-0! It's not easy to come here [as the away side] and take points away."
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