Jelavic, 30, has signed a two-year deal at Upton Park, where he will link up again with manager Slaven Bilic after playing for the West Ham boss during his time in charge of Croatia.
He follows Alex Song and Victor Moses, both on season-long loan deals, into Upton Park on a busy day for the club. The Hammers are also trying to sign Antonio from Nottingham Forest, although talks are said to have stalled.
Jelavic has scored one Championship goal for the Tigers this season after scoring eight goals the Premier League last season as Hull were relegated.
Jelavic, who underwent surgery on a long-term knee problem earlier this year, admits the lure of playing under Bilic was part of the attraction in moving to east London.
“I’ve played for Slaven before for the Croatia national team and he is a great manager and a great man,” he told West Ham TV.
“I am really happy and delighted to be working with him and all his staff again. I am very excited to be signing for West Ham United, especially as it is a big club and I didn’t know until a couple of days ago that I would have the opportunity to come here.