The 50-year-old Portuguese, who led the club to unprecedented success between 2004 and 2007, succeeds interim coach Rafael Benitez and has signed a four-year contract.
His Stamford Bridge comeback was one of the worst-kept secrets in European football, with weeks of media speculation culminating in his admission on Spanish television late on Sunday that he had agreed to move back to west London.
In an interview with Chelsea TV, the club’s in-house television channel, Mourinho said his time at the club had left a deep impression that endured throughout his subsequent tenures at Inter Milan and Real Madrid.
“It is exactly the same message, but now I can say I am one of you (supporters) and that makes a little bit of a difference,” he said.
“I never hide that in my career in football I had two great passions — Inter and Chelsea, and Chelsea is more than important for me.
“It was very hard to play against Chelsea. I did it twice, which was not so bad. Now I promise exactly the same things I promised in 2004, but with the difference we have (now), which is I am one of you.”
Announcing the news of Mourinho’s reappointment, Chelsea chief executive Ron Gourlay heralded his “success, drive and ambition”, and said he was the “outstanding candidate”.
Mourinho arrives from Madrid, where an initially fruitful three-year spell ended in acrimony following an unsuccessful league title defence, a semi-final exit in the Champions League, and the alienation of senior players including goalkeeper Iker Casillas.
Prior to joining Madrid, he enjoyed a stunning two-season stint at Inter that culminated in an unprecedented sweep of Serie A, Italian Cup and Champions League trophies in 2010.
He is one of only four coaches to have won the European Cup with two different clubs, having first tasted glory in the competition with Porto in 2004.
His subsequent spell as Chelsea manager saw the club emerge as the dominant force in English football.
Backed by the investment of billionaire Russian owner Roman Abramovich, Mourinho won back-to-back Premier League titles in 2005 and 2006, as well as two League Cups and the 2007 FA Cup.
“He’s a great manager,” said midfielder Frank Lampard, a survivor of Mourinho’s first spell at Stamford Bridge.
“He took my game on a million miles, and my personality in terms of football on a million miles. A lot of the reasons I moved on in the game are because of him.”
Popularly referred to as ‘the Special One’, in reference to a brash remark from his very first Chelsea press conference, Mourinho is an outspoken figure who has became renowned for his clashes with officials and opposition coaches.
Former Chelsea striker Tony Cascarino said Mourinho’s return would help fill the void created by the shock retirement of long-serving Manchester United manager Alex Ferguson.
“We’ve lost one great manager in Fergie, but we’ve got another one coming back into the Premiership,” he told Sky Sports News.
Ron Harris, Chelsea’s record appearance-maker, said: “I feel the supporters will be more than happy, so it’s a pat on the back for Roman Abramovich for bringing him back to Chelsea Football Club.”
Mourinho succeeds Benitez, who led Chelsea to Europa League glory and a top-four finish this season after replacing sacked Champions League-winning coach Roberto Di Matteo last November. Benitez has already agreed to join Napoli next season.
Mourinho has brought coaches Rui Faria, Silvino Louro and Jose Morais with him from Madrid and will be officially presented to the media next Monday.
After a pre-season tour of Asia in July, Mourinho’s first chance of major silverware will arrive when Chelsea tackle Bayern Munich, coached by his former Barcelona sparring partner Pep Guardiola, in the European Super Cup final in Prague on August 30.