The 52-year-old Portuguese had been in his second spell at the club, taking charge in June 2013.
Chelsea finished eight points clear last season and also won the League Cup, but have endured a terrible start to their defence, losing nine of their 16 league games so far.
Mourinho’s final match was Monday’s 2-1 defeat at leaders Leicester City.
Pep Guardiola, Guus Hiddink, Brendan Rodgers and Juande Ramos have all been touted as possible successors.
He leaves with Chelsea just a point off the relegation zone and 20 points behind Leicester.
Champions League qualification via one of the top four spots in the Premier League looks almost impossible.
However, the Blues are through to the last 16 of European football’s biggest club competition, where they will face French champions Paris St-Germain in February and March.
Mourinho only signed a new four-year contract on 7 August and is Chelsea’s most successful manager.
He won three Premier League titles, two of those in his first spell between 2004 and 2007.
The Portuguese has also won the FA Cup, in 2007, and the League Cup three times – in 2005, 2007 and 2015.