The Nigeria striker joins from Turkish side Fenerbahce until the summer, when West Ham will have the option of making the deal permanent.
Emenike played in Russia and Turkey when Slaven Bilic was managing other clubs in the same leagues, and the 28-year-old is looking forward to working with his new boss.
“I’m very, very happy and very, very excited that I’m here,” he told West Ham TV. “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.
“I’ve known him [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, who was the top scorer when Nigeria won the 2013 Africa Cup of Nations, joins up with international team-mate Victor Moses at the Boleyn Ground.