The Serbia midfielder, who left Stamford Bridge as part of the deal which saw David Luiz move to west London from Benfica three years ago, is now back with the Blues in a deal reported by the Portuguese club to be worth 25million euros (£20.75million).
He will compete with a host of top-class midfielders for a place in Mourinho's side but the manager is delighted to welcome him back.
"I am very pleased that Nemanja is returning to Chelsea," said Mourinho.
"He has grown as a player in Portugal and has become a fantastic all-round midfielder. I am sure he will become a very important member of the squad and help us to achieve our ambitions."
The 25-year-old struggled to make an impact during his first spell at the club, spending a season on loan at Dutch side Vitesse Arnhem before heading to Portugal.
And he will be up against Ramires, John Obi Mikel, Frank Lampard, Luiz and Michael Essien for the "holding" midfield places in Mourinho's squad, while Marco van Ginkel is working his way back from a long-term injury.
There is a similar embarrassment of riches in the attacking midfield department, with two-time club player of the year Juan Mata unable to command a place and Kevin de Bruyne seemingly Wolfsburg-bound as Mourinho favours Eden Hazard, Oscar and Willian.
Matic first came to prominence during his time at MSK Kosice and was signed by then-Chelsea boss Carlo Ancelotti as a promising youngster.
Although his career did not take off, Matic impressed during his time at Benfica and played in the Portuguese side's 2-1 defeat to Chelsea in the Europa League final last season. He has won nine caps for his country.