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.