Paris-born Demel joins the Hammers after signing a two-year contract with the option of an extra year.
A member of his adopted nation’s 2010 World Cup squad, Demel will add even more experience to the West Ham squad having been capped 26 times whilst also playing for two of Germany’s biggest clubs, representing Hamburg and winning the title with Borussia Dortmund beforehand.
Speaking about his move to London, Demel told the club’s official website: "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.
After having a fairly successful decade in Germany, Demel conceded that it was time for a change of scenery as he searched for a new a challenge and despite being for his time in the Bundesliga, he is excited by the next chapter in his career.
"I had 10 years in Germany and I won the title,” he explained. “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."