Ten Hag, who had his contract extended in the summer, had just four wins from 13 games to start the new season.
Ruud van Nistelrooy will take over on an interim basis and is expected to be in charge for the Carabao Cup tie against Leicester City at Old Trafford on Wednesday.
Sources told ESPN that United bosses made the decision to sack Ten Hag after losing faith that the Dutchman would be able to turn things around after a poor start to the new campaign. After earning just 11 points from nine league games, there are already concerns that United are too far adrift to qualify for next season's Champions League.
Results and performances in the Premier League at the end of last season were also taken into account.
United finished eighth in the table last season and are languishing in 14th after defeat at the London Stadium on Sunday.
#ten_hag #man_united