The midfielder has penned down a three-year deal at London Stadium after ending his 17 year spell at Arsenal.
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Wilshere becomes new manager Manuel Pellegrini’s fourth signing after Ryan Fredericks, Issa Diop and Lukasz Fabianski.
The former Bournemouth loanee was left out of Gareth Southgate’s squad for the World Cup.
Wilshere’s career has been plagued by injuries but made 38 appearances for the Gunners last season, more than the previous three seasons combined and most since 2010-11.
The 26 year old’s last appearance for Arsenal came in 5-0 win over Burnley in Arsene Wenger’s final home game.
A West Ham fan as a boy, Wilshere has been linked with the club since the summer of his loan move to Bournemouth ahead of the 2016-17 season.
He said: “It feels good, it feels special.
“Many people know that I’ve had a special bond with this Club growing up.
“People will have seen the picture of me in a West Ham shirt and I’ve got good memories of my childhood supporting West Ham, watching them at Upton Park. It feels good and of course my family and a few of my friends are West Ham fans so everyone’s buzzing.”
West Ham begin their Premier League campaign under Pellegrini with a trip to Liverpool on August 12.