The 52-year-old's second spell in charge at Stamford Bridge comes to an end just seven months after he led the Blues to the English title.
Mourinho's men also won the League Cup last season, but their form has nosedived in 2015-16.
In a statement on the club website, Chelsea said: "All at Chelsea thank Jose for his immense contribution since he returned as manager in the summer of 2013.
"His three league titles, FA Cup, Community Shield and three League Cup wins over two spells make him the most successful manager in our 110-year history. But both Jose and the board agreed results have not been good enough this season and believe it is in the best interests of both parties to go our separate ways.
"The club wishes to make clear Jose leaves us on good terms and will always remain a much-loved, respected and significant figure at Chelsea."
According to BBC Sport, Mourinho, who will be paid for the rest of the season, was told of his departure this afternoon in a meeting with the club's board.
The latest reports suggest that Chelsea could target a move for either Pep Guardiola or Diego Simeone at the end of the season and in the meantime appoint an interim manager for the rest of the season.