Pogatetz, who can play at left-back or in the centre of defence, has more than 50 caps for his country and experience of the Premier League from five seasons with Middlesbrough.
The powerful 30-year-old had been playing in Germany for the last two-and-a-half years but is delighted to be returning to England.
"I feel very good to be here and I am very excited about playing for West Ham," he 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.
"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."