The Ivory Coast international has signed a two-year deal with the option of a further year. He is likely to play on the right side of defence.
Speaking to the official site Demel admitted that the ambition shown by the club was one of the factors in deciding to sign for West Ham.
“I am very excited to join West Ham. I want to help the team go back to the Premier League. This is my goal.
“I have some friends here already like Freddie Piquionne and Carlton Cole, and London is a great city. I am very happy to be here. I know the players here want the same as me. Everyone is determined to make it a successful season and this is a great opportunity.
“I had ten years in Germany and I won the title. But it is time to change and show what I can do in another country. I will do my best.
“It is a good stadium and I am also looking forward to the future. I would like to go to the new stadium with West Ham. Something big is happening here and I want to be a part of an exciting time in the club’s history. I know the fans are very passionate.”