Manchester-born Morrison, who turns 19 on 2 February, has penned a three-and-a-half-year contract with the Championship side and will be eligble to make his Hammers debut this Saturday when West Ham host Millwall in what promises to be a fiery East London derby.
Morrison, an England youth international, was a member of 2011's FA Youth Cup-winning side and made three first-team appearances during his time at United, all in the Carling Cup competition.
Speaking to West Ham's official club website, Morrison said he's looking forward to proving himself at senior level.
"I'm really pleased to have signed and I'm very happy," he said. "The move has happened very quickly and I'm looking forward to hopefully moving up to the Premier League with West Ham soon.
"I'm an attacking player and I'm hoping to get the fans on their feet.
"I met with Sam Allardyce this morning and he welcomed me here. I also played with Robert Hall in the England team and I know him well so that will be really helpful to have someone here that I know."
Ravel leaves United despite being offered a contract in mid-January to remain at Old Trafford. That revelation came after Newcastle reportedly tried to lure Morrison to the North East with an offer United turned down.
"His agent has been working hard to get him another club,” Sir Alex told the press on 14 January. “We’ve offered him terms which he has refused, so that is where it is. His demands are unrealistic as far as we are concerned. We’ve rejected an offer from Newcastle and it is now down to how that