Austrian international Pogatetz, who has arrived on a short-term deal from Bundesliga outfit Wolfsburg, revealed his delight with the surprise move.
He told the club's official website: "I feel very good to be here and I am very excited about playing for West Ham.
"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 30-year-old, who can also play at left-back, made 156 appearances for Boro between 2005 and 2010.