The football club has announced that Greenwood is suspended from training or playing with the Premier League giants after the allegations were reported on a woman's Instagram account.
Manchester United player Mason Greenwood has been arrested on suspicion of rape and assault following allegations on social media.
The police did not name Greenwood but a statement about the investigation was provided on Sunday after inquiries about the 20-year-old forward.
“Greater Manchester Police were made aware earlier today of online social media images and videos posted by a woman reporting incidents of physical violence," the force said in a statement.
“An investigation was launched and following enquiries we can confirm a man in his 20s has since been arrested on suspicion of rape and assault.
“He remains in custody for questioning. Enquiries are ongoing."
Manchester United said it does not condone violence of any kind. Greenwood has since been suspended from training or playing with the Premier League giants "until further notice".
The allegations were posted early Sunday morning on the Instagram account of a woman who uploaded images of bruising to her body and bleeding from her lip.
A voice note purporting to be of an attack was also posted.
The posts were all deleted from the social media site but were widely shared.
Under English law, alleged and actual victims of sexual offences cannot be named.
Greenwood has yet to make any public comment.
Nike, one of Greenwood's sponsors, expressed its unease.
“We are deeply concerned by the disturbing allegations and will continue to closely monitor the situation," the sportswear firm said in a statement.
Greenwood, who progressed through the United academy into the first team, has scored six goals this season. He extended his contract last year through 2025.
Greenwood made his England debut in September 2020 but was sent home from Iceland for a disciplinary breach after the game. He hasn't played since for Gareth Southgate's side.