The club said he had left with immediate effect, adding they “would like to thank Jose for his work during his time at Manchester United and to wish him success in the future”.
A new caretaker manager will be appointed until the end of the current season, while the club searches for a new full-time manager.
Mourinho’s side lost 3-1 to Liverpool on Sunday, as two deflected Xherdan Shaqiri strikes took the Reds to the top of the table.
In contrast, the result left United 11 points off the top four, who qualify for the Champions League, and 19 points behind top place.
The Portuguese, 55, had been in charge at Old Trafford for two and a half years, but, despite winning the EFL Cup and the Europa League in his first full season, he has rarely appeared settled.
The team, which is expected to at least challenge for the league title every season, had a best finish of second under him, last season, but they were 19 points behind winners Manchester City.