The Hammers have been scouring the market for a striker in January and have landed the Nigeria international, who says he is "very excited" to move to the Premier League
The 28-year-old's move was confirmed on Sunday, with the Premier League club retaining the option to make the switch permanent at the end of the campaign.
A 37-time Nigeria international, Emenike has already spent time on loan at Al Ain this season, scoring nine goals in 15 games.
"I'm very, very happy and very, very excited that I'm here," Emenike told the club's official website.
"I have always known West Ham as a great club. I've always been watching them. West Ham's a great club and I'm happy to be here.
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"I've known him [Slaven Bilic] for years. When he was at Lokomotiv Moscow, I was at Spartak Moscow, then he came to Besiktas and I was in Fenerbahce, so we've known each other for a long time. I'm happy to be here and happy to play for him.
"We've followed each other's careers and thank God we are here today and we are here together."
Emenike scored regularly for Karabukspor and Spartak Moscow before his move to Fener for a second time in mid-2013.
Two years earlier, he had joined the club but was sold due to a match-fixing scandal.
Emenike scored 12 league goals in his first season at Fenerbahce but struggled in 2014-15, leading to his loan moves.