Mourinho has signed a four-year contract and will return to the club where he won two Premier League titles, an FA Cup and two League Cups between 2004 and 2007.
Ron Gourlay, Chelsea FC Chief Executive, said: 'I am delighted to welcome Jose back to Chelsea. His continued success, drive and ambition made him the outstanding candidate.
'It is our aim to keep the club moving forward to achieve greater success in the future and Jose is our number one choice as we believe he is the right manager to do just that.
'He was and remains a hugely popular figure at the club and everyone here looks forward to working with him again.'
Since leaving Chelsea in 2007, the 50-year-old Portuguese has worked in Italy with Inter and Spain with Real Madrid, lifting three league championships, two domestic cups and the Champions League.
Source: Chelsea FC