Chelsea have completed the signing of former midfielder Nemanja Matic from Benfica for a fee believed to be in excess of £20m.
The 25-year-old left Chelsea in 2011 to move to Benfica as part of the deal that saw David Luiz arrive at Stamford Bridge.
He has signed a five-and-a-half-year contract with Chelsea, keeping him at the club until the summer of 2019.
Matic, who arrived in London on Tuesday night ahead of putting pen-to-paper on his contract with Chelsea on Wednesday afternoon, said he is delighted to return to Chelsea.
"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."
The Serbian midfielder was initially signed by then-manager Carlo Ancelotti for Chelsea in 2009 from MSK Kosice at the age of 21.
After finding first-team opportunities limited at Stamford Bridge, Matic moved to Benfica and his new manager Jose Mourinho insists he is returning to the club as a far better player.
"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."
Matic will wear the Number 21 shirt for Chelsea for the remainder of this season.
The Serbia international confirmed his departure from Portugal by announcing that last weekend's win over Porto was his final game for the club.
He told A Bola: "I want to thank everyone at the club and all the fans. Benfica will forever be in my heart.
"I played here for two-and-a-half years, gave my best at every training session, and now it is the right moment to leave.
"I want to improve, I want to play in a better league and I want to go to Chelsea, a club I also love.
"I would also like to thank Benfica's president, he did everything he could to keep me, but I wanted to leave.
"I insisted on leaving because I think this is the right moment."