The announcement comes just six days before Spurs are due to face Manchester City in the EFL Cup final, where they could win their first trophy since 2008.
However, Tottenham have won just one of their last six matches across all competitions, suffering an early exit from the Europa League and slipping five points behind West Ham United in the top-four race in the process.
Spurs have lost 10 Premier League games already in 2020-21, and nine times since the turn of the year across all competitions.
A statement on the club's official website read: "The club can today announce that Jose Mourinho and his coaching staff Joao Sacramento, Nuno Santos, Carlos Lalin and Giovanni Cerra have been relieved of their duties."
Chairman Daniel Levy added: "Jose and his coaching staff have been with us through some of our most challenging times as a club. Jose is a true professional who showed enormous resilience during the pandemic.
"On a personal level I have enjoyed working with him and regret that things have not worked out as we both had envisaged. He will always be welcome here and we should like to thank him and his coaching staff for their contribution."
News of Mourinho's departure comes less than 12 hours after Tottenham confirmed that they are part of a 12-team breakaway group of European clubs planning to form a controversial but lucrative new European Super League.
Former Spurs midfielder Ryan Mason will take first-team training today and could be in charge for Wednesday's Premier League match against Southampton, assisted by ex-Charlton Athletic and Huddersfield Town boss Chris Powell.
Mourinho took charge of Tottenham in November 2019 and leaves having won 44, drawn 19 and lost 23 of his 86 matches at the helm.
RB Leipzig boss Julian Nagelsmann has been tipped as the early favourite to replace Mourinho at the Tottenham Hotspur Stadium.