Chelsea have completed the signing of midfielder Nemanja Matic from Benfica.
In a statement to the Portuguese Stock Exchange, Benfica confirmed that the deal is worth €25 million.
Chelsea confirmed that Matic has put pen to paper on a deal that will keep him at Stamford Bridge until 2019.
The 25-year-old told Chelsea's official website: "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."
Chelsea boss Jose 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."
Matic will wear the number 21 shirt, previously worn by Marko Marin, who is currently on loan with Sevilla.
The move marks the Serbian international's second spell at Chelsea; he signed for the Blues back in 2009 in a deal believed to worth around €2 million, before he joined Benfica as part of the deal which saw defender David Luiz head to west London in 2011.
He made 56 league appearances for the Portuguese giants, and was a finalist for the FIFA Puskas goal of the year award, but missed out to Zlatan Ibrahimovic's wonderstrike against England.
Matic made his final appearance for Benfica at the weekend, helping his former side secure a 2-0 win over fierce rivals Porto.