The Hammers said their "thoughts and prayers" are with Antonio after an incident in Essex on Saturday, later updating saying the 34-year-old is "conscious and communicating" in hospital.
"West Ham United can confirm that Michail Antonio is in a stable condition following a road traffic accident this afternoon in the Essex area," West Ham said.
"Michail is conscious and communicating and is currently under close supervision at a central London hospital."
West Ham added for Antonio's privacy to be respected while ensuring further updates will be provided "in due course".
Earlier on Saturday evening, West Ham published a statement saying Antonio "has today been involved in a road traffic accident".
"The thoughts and prayers of everyone at the Club are with Michail, his family and friends at this time. The Club will issue an update in due course," they added.
Antonio joined West Ham from Nottingham Forest in 2015 and has made a total of 322 appearances for the Hammers.
The English-born Jamaica international has scored 83 goals for the east London club in all competitions.
Antonio began his career with Tooting & Mitcham United, breaking into their senior squad during the 2008/09 season.
He quickly signed for Reading, where he would spend three years before joining Sheffield Wednesday in 2012 after a successful loan spell.
West Ham – currently 14th in the Premier League – are next in action at home to Wolves on Monday.