Matic 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.
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.
Benfica claim to have received a fee of €25 million (£20.75 million) for the player, who is now ready to grasp his second chance at Stamford Bridge.
"I am very happy for this opportunity to be back at this club," he told chelseafc.com
"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."
Blues' manager Jose Mourinho welcomed the signing and believes the player, capped nine times by Serbia, has developed since he was last at the club.
"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."
- Sports & Recreation
- David Luiz
- Vitesse Arnhem
- Carlo Ancelotti