The 20-year-old has signed a four-year deal with the Hammers.
West Ham United's latest signing Ashley Fletcher has told the club's official website that a key motivation for his move to the Olympic Stadium was the Hammers' tradition of developing young talent into first-team stars.
Fletcher has ended speculation about his future by leaving Manchester United to sign a four-year deal at the Hammers, who have beaten off a host of challengers to secure his signature.
The 20-year-old striker becomes the Irons' fourth major signing of the summer after midfielder Havard Nordveit and Sofiane Feghouli and winger Gokhan Tore.
Fletcher never made a first-team appearance for United but attracted the attentions of a host of clubs as a result of an impressive season on loan at League One Barnsley last term, for whom he scored nine times in 27 appearances.
His general displays have seen him emerge as a hotly-tipped future star, and he admitted West Ham's tradition of bringing through talented youngsters was a key factor in his decision to join the Premier League side.
He told the club's official website: "I am very excited to be here. West Ham are a massive club and I am just happy that I can come and further my development here at West Ham.
"I think it is a club with a lot of ambition. They have a lot of good players and it was a special final season. They managed to finish the campaign very well.
"West Ham are a club built on traditions of giving youngsters a run and hopefully I can get my chance as well."
Current Hammers captain Mark Noble and young midfielder Reece Oxford are two of the current first-team members to have come through West Ham's academy, whilst former England stars including Frank Lampard, Rio Ferdinand, Jermain Defoe and Michael Carrick all made their name at the club.
Fletcher will be hoping to be given the opportunity to begin following in the footsteps of those illustrious names under Slaven Bilic this season.