The 20-year-old striker has joined the Hammers on a four-year deal after his contract at Old Trafford expired at the end of June.
The Red Devils will be due compensation but West Ham are yet to finalise those terms with the Manchester club.
"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," Fletcher told West Ham's official website.
"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."
Extremely honoured to sign for @whufc_official .Looking forward to this next chapter at such a special club https://t.co/11z0exN23U
— Ashley Fletcher (@AshFletcher___) July 12, 2016