He has now departed the Gunners after a 17 year association.
He said, "It feels good, it feels special".
He becomes the latest recruit in what looks set to be a significant summer of investment for West Ham and new manager Manuel Pellegrini.
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"I think I must have been about six or seven, my whole family were West Ham fans, I was a West Ham fan, still have a soft spot for them of course".
"Many people know that I've had a special bond with this Club growing up".
On top of this, a picture of a young Wilshere wearing a Hammers shirt was leaked in 2013, which makes this move even more well-suited to the 26-year-old's career.
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Mario Husillos, director of football at West Ham, said he and Pellegrini had made Wilshere's signing a priority once they knew he would be available.
Wilshere might have played more games last season (38) than in any campaign since 2010/11 but his reputation appears to be at rock bottom.
Wilshere, who supported West Ham as a boy, will now fly out to Switzerland to join his new team-mates on their pre-season training camp in Bad Ragaz.
The Ukraine worldwide is valued at €20m (£17m/$23m) by the Bundesliga side, per ClaretandHugh, but it is as yet unclear if the club will try to sign both players, or merely one.
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The injury-plagued Wilshere proved his fitness for the Gunners last season, making 29 starts in all competitions.