The defensive-minded midfielder becomes the ninth player through the door at West Ham during a productive summer window for Manuel Pellegrini.
It has certainly been a productive summer as Pellegrini bids for his West Ham side to improve on their 13th-placed finish last term.
Perez scored seven goals in 21 appearances for the North London club.
"I'm very happy to be here at such an historic club", he told the club's official website.
Arsenal are said to be keen to cash in on Perez and Emery has reportedly warned both the Spaniard and fellow striker Danny Welbeck that they will have minimal game-time if they stay at the club this summer.
'I'd like to thank all of the people, the whole club, for having me'.
Browns trade WR Corey Coleman to Bills for draft pick
We've all seen the return that Cleveland received from their trade with the Buffalo Bills , yet it's still too shocking. As a rookie, Coleman started all 10 games in which he played, catching 33 passes for 413 yards and three touchdowns.
Steven Seagal appointed by Russian Federation as special envoy to the U.S.
The ministry further stated the Seagal's unpaid role will be similar to the United Nations' goodwill ambassador. Russian Federation is like a second "home" to Seagal.
Chelsea Keen On Liverpool Target Nabil Fekir But Europa League Negative Factor
Nabil Fekir has reignited speculation over his future by suggesting he could still move away from Lyon in this transfer window. Nabil Fekir will not feature for Lyon against Chelsea at Stamford Bridge this evening as talk of a move to London rumbles on.
"I wish you all the best for the future and I hope you can go on and achieve great things".
This season he will show if he can make it in the top tier of English football or whether he will continue to struggle, but if he is able to repeat his form from Deportivo at West Ham, he could be seen as a steal for only £5m.
West Ham also sold one player on deadline day, shipping 18-year-old Portuguese midfielder Domingos Quina to Watford.
"His style is different from the other strikers we have available to us, and that gives the manager more options for our attacking play".