The Serbian international has signed a five-and-a-half year contract at Stamford Bridge following his spell at Benfica, only three years after he left west London
The Blues were on the brink of agreeing a deal worth €25 million for the Serb on Monday after holding talks with the Portuguese giants in Lisbon.
Matic travelled to London on Tuesday to undergo a medical, and has now signed a five-and-a-half year contract at Stamford Bridge.
"I am very happy for this opportunity to be back at this club," Matic told the club's official website. "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."
The 26-year-old joined Chelsea for just €1.8m in 2009 but left two years later as part of the deal which brought David Luiz to west London in January 2011.
Jose Mourinho has long considered a top-class defensive midfielder his most urgent transfer target and with his physique, technical ability and athleticism, Matic is believed to fit the bill perfectly.
Mourinho added: "I am very pleased that Nemanja is returning to Chelsea. 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 return of Matic will be financed in large part by the €18m sale of Kevin De Bruyne to Wolfsburg, which is expected to be announced imminently.