Chelsea appoint former Dutch boss Guus Hiddink
The 69-year-old also managed the Blues on a temporary basis for the final three months of the 2008-09 campaign.
"I am excited to return," said the Dutchman. "Chelsea is one of the biggest clubs in the world but is not where it should be at the moment.
"However, I am sure we can all turn this season around."
Mourinho was sacked on Thursday with champions Chelsea 16th in the Premier League table, one point above the relegation zone.
Hiddink watched Saturday's visit of Sunderland from the stands, sat next to Chelsea owner Roman Abramovich and former Blues striker Didier Drogba.
Coaches Steve Holland and Eddie Newton were in charge of the team for the game against the Black Cats.
"It's a fantastic appointment. I'm very happy about it. This club needs an experienced manager at the helm and Guus Hiddink is clearly that," said Holland.
Chelsea technical director Michael Emenalo said a "palpable discord between manager and players" had contributed to Mourinho's dismissal, the second time he has been sacked by the club.
Bayern Munich manager Pep Guardiola, who is set to reveal on Saturday if he will stay at the German club beyond the end of the season, is among the possible permanent replacements.
Atletico Madrid's Diego Simeone and out-of-work Carlo Ancelotti, who managed Chelsea for two seasons from June 2009, have also been suggested as potential candidates for the full-time role.