Mourinho enjoyed a hugely successful spell in charge at Stamford Bridge between 2004 and 2007, landing two Premier League titles, one FA Cup and two League Cups.
The Portuguese boss has since led Inter Milan to Champions League and Serie A glory, twice, as well as helping Real Madrid secure the Copa del Rey title in 2011 and the La Liga crown the following season.
Mourinho left Real Madrid at the end of the 2012/13 Spanish season having failed to land any major silverware and had been linked with a switch back to Chelsea following the dismissal of Roberto Di Matteo in November.
It has now been confirmed that Mourinho has rejoined Chelsea and has agreed a four-year deal at Stamford Bridge following the exit of interim manager Rafael Benitez at the end of last season.
Chelsea chief executive Ron Gourlay said on the club's official website: "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."