The 31-year-old center back has welcomed the chance to return to England and hopes to aid the mid-table club in its quest to stay in the top flight.
West Ham has signed Wolfsburg defender Emanuel Pogatetz on loan until the end of the season.
The 31-year-old is thought to have also attracted interest from Fulham, but he will now spend the next five months at Upton Park after finding first-team opportunities hard to come by in Germany. The former Middlesbrough player has also expressed his delight at returning to England, after spending five years plying his trade in the Premier League and League Championship from 2005-2010.
"I feel very good to be here and I am very excited about playing for West Ham," Pogatetz told the club's official website. "I am looking forward to the opportunity to play for the club and help it get good results. 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."
The Austria international, who has been capped 52 times, added: "First of all when I heard that West Ham wanted me, I thought it was a club I wanted to play for because they are a very traditional club, with passionate supporters.
"Secondly, it was good for to have the opportunity to come back and play in England. This is the football I want to play and I have missed playing here, I want to show what I am still capable of after five years playing in Middlesbrough."