The first images of the new signing were seen in Wilshere's Instagram story, where he snapped a shot of Perez in the Hammer's training kit with a huge smile on his face.
Sanchez, who has over 80 caps for his country, spent past year on loan at Espanyol.
Pellegrini has overhauled West Ham's squad this summer breaking the club's transfer record to sign Felipe Anderson from Lazio while signing Jack Wilshere, Fabian Balbuena, Andriy Yarmolenko, Lukasz Fabianski, Ryan Fredericks, Issa Diop and youngster Xande Silva.
The 29-year-old has signed a three-year deal with the Hammers, having spent the last two seasons with the Gunners.
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Champions League hat-trick aside, Perez never got going for the Gunners after joining from Deportivo la Coruna for £17million, and will therefore undoubtedly retain "flop" statue forever more.
Perez said: "I'm very happy to be here at such an historic club".
'I also want to give my thanks to my teammates and to the staff of this club for giving me the opportunity to make my debut in one of the best leagues in the world and win the FA Cup.
Chicharito added: "The manager called me before the World Cup because he knew my situation was hard at the end of last season, and he made me feel very good".
Perez managed to score eight goals in 35 matches for Deportivo who have been relegated into the Segunda division in Spain last season.