Nikica Jelavic became countryman Slaven Bilic's third deadline day signing, following the acquisitions of Alex Song (permenantly) and Victor Moses (loan move).
The 30 year old seemed delighted to be once again playing under his former national team boss, following a mixed 18 month spell at Hull City.
"I didn't know until a couple of days ago that I would have the opportunity to come here" the striker told West Ham's official website.
"Of course it is always a target to play in the Premier League, so when I found out that West Ham wanted me there was no doubt that I wanted to come here.
"I've played for Slaven before for the Croatia national team and he is a great manager and a great man.
"I am really happy and delighted to be working with him and all his staff again".