The Serbia midfielder, who left Stamford Bridge as part of the deal which saw David Luiz move to west London from Benfica three years earlier, agreed a deal reported by the Portuguese club to be worth 25million euros (£20.75million).
“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 had 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 faced a battle to hold down a place against fellow midfielders like Ramires, John Obi Mikel, Frank Lampard, Luiz and Michael Essien.
Matic told chelseafc.com: “I am very happy for this opportunity to be back at this club.
“I feel very good because of that and just want to say I will give my best to help the team and make the Chelsea fans happy.”
Matic went on to enjoy a relatively impressive three years in his second stint at Stamford Bridge before moving on to join his former boss Mourinho at Manchester United.