The Austria international, who played for Middlesbrough between 2005 and 2010, has joined the Hammers from Wolfsburg.
"I feel very good to be here and I am very excited about playing for West Ham," he told whufc.com.
"I am looking forward to the opportunity to play for the club and help it get good results."
He added: "I will be doing my best to play well and hope I can play a part in the club achieving the targets for this season."
He has only made eight Bundesliga appearances for Wolfsburg this season.