Chelsea did not reveal the transfer fee but British media reports said they had paid 21 million pounds to re-sign the Serbian international, who left Stamford Bridge as a makeweight in the deal that brought defender David Luiz to Chelsea in January 2011.
"I am very happy for this opportunity to be back at this club," Matic, 25, said on the club website (www.chelseafc.com).
Chelsea manager Jose Mourinho added: "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."
Chelsea initially signed Matic from MSK Kosice in 2009 and he made three substitute appearances in the 2009-10 season.
The towering central midfielder's career appeared to take a downturn when he was loaned by Chelsea to Dutch side Vitesse Arnhem in 2010 before being sold to Benfica.
He also pulled out of the national team in 2012 after falling out with former Serbia coach Sinisa Mihajlovic.
Matic, however, proved his worth at Benfica where he quickly established himself as an automatic starter and a lynchpin between defence and attack.
He also returned from the international wilderness in September and played in the final stages of Serbia's failed 2014 World Cup qualifying campaign under Mihajlovic.
"The time has come for me to go to a league of higher quality, as I want to improve and I also wish to thank everyone at Benfica for three wonderful years I spent there," Matic said after his final game for the Portuguese club, a 2-0 derby win over Porto on Sunday.
"I am returning to the best league in the world and also to a club I cherish very much. Being asked to return to Chelsea by a coach like Jose Mourinho is no mean feat."
(Additional reporting by Zoran Milosavljevic,; Reporting by Toby Davis; editing by Ed Osmond)
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