Moyes, 50, was dismissed at a meeting with executive vice-chairman Ed Woodward at the club's Carrington training ground this morning, BBC reports.
Moyes left Everton to sign a six-year deal with Man Utd in 2013 but the Premier League champions have been playing poorly since he took over as manager.
United, who lie seventh in the table with four games remaining, are guaranteed to record their lowest points tally in the Premier League.
Ryan Giggs has been named as interim manager . Now the big question is who's next?
Netherlands coach Louis Van Gaal, Borussia Dortmund boss Jurgen Klopp, Atletico Madrid's manager Diego Simeone and Paris St-Germain's Laurent Blanc have already been tipped as possible successors.