The former Wigan Athletic midfielder has signed a three-year deal at the KC Stadium after becoming their second deadline day signing
The 27-year-old has signed a three-year contract with Steve Bruce's side after falling down the pecking order at Upton Park.
He becomes the Tigers second signing on deadline day after they also agreed a deal to bring Abel Hernandez to the club.
"I'm delighted to welcome Mohamed to the club," Bruce told Hull's official website.
He added: "His experience, both at international level and in the Premier League will be invaluable for us as we look to cement our place in the top-flight."
Diame moved to England in 2009 after joining Wigan Athletic before moving to West Ham in 2012 on a free transfer.
He went onto make 71 Premier League appearances for the Irons, scoring seven goals.