The Italian forward signs from Juventus with the option of a permanent move
West Ham have bolstered their attacking before the end of the transfer window on Wednesday with the signing of Simone Zaza on a season-long loan.
The Italian forward, who featured for the international side during the European Championships in France, joins the Hammers from Juventus with the option of a permanent deal - in which case the fee would rise from over £4m to £20m.
"I want to thank everyone for welcoming me including the fans and everyone at the club," he told the club's official website. "I wanted to start a new experience and I feel this is the right choice for me.
"I have been fortunate enough to have already played at the new Stadium with a different shirt. It is a beautiful stadium and I felt the warmth of the fans and this was a big reason why I wanted to join the club.
"I want to become a better player and I know I can do that at West Ham. Unfortunately we could not qualify for the Europa League, but I am hoping I can help the club do that next season."