Players flooding in
The youngster has become the Hammers fourth signing of the summer, with Havard Nordtveit, Sofiane Feghouli and Gokhan Tore already signing.
The 20-year-old is touted as one of the young hot prospects in the country, having come up through the ranks at United, he was loaned to Barnsley.
After joining the Yorkshire club in January 2016, he impressed many, with scoring nine goals including one against Millwall in the League One play-off final.
There was a lot of interest surrounding the England U20 international, but Fletcher has said he is relishing the opportunity to show his best form.
He told whufc.com “I am very excited to be here" as "West Ham are a massive club" and is "just happy that I can come and further my development here at West Ham."
Fletcher concluded: “I think it is a Club with a lot of ambition" with a "lot of good players" and a "special final season" who are a "club built on traditions of giving youngsters a run and hopefully I can get my chance as well.”
Delighted to be there
The Hammers have been busy in the past few days, having completed the loan signing of Tore on Monday.
The winger came in from Besiktas on loan initially, with the view for a permanent transfer, and he also shared his delight at his move.
Tore said "I am very happy to be at West Ham" and "watched many games that they played last season and saw how passionate the fans are" and is "looking forward to playing in front of them."
West Ham United will take on the Carolina RailHawks at the WakeMed Soccer Park on Wednesday, July 13 with kick-off at 1am BST.