Chelsea have completed the £21m signing of Benfica midfielder Nemanja Matic on a five-and-a-half-year deal.
The 25-year-old left Stamford Bridge for Portugal three years ago.
"I am very happy for this opportunity. I just want to say I will give my best to help the team and make the Chelsea fans happy," said Matic.
Chelsea boss Jose Mourinho said: "He has grown as a player in Portugal and has become a fantastic all-round midfielder."
The Serbia international was valued at less than £5m in January 2011 when he left Stamford Bridge as a makeweight in the deal that saw defender David Luiz join Chelsea from Benfica.
Mourinho quickly identified the defensive midfielder as a key target on his return to Stamford Bridge and the club had no issue with re-signing a player they previously let go.
Chelsea will offset the cost of signing Matic with the £16m they will receive from the proposed sale of Kevin De Bruyne to Wolfsburg.
Matic joined Chelsea from Slovakian club MFK Kosice on a four-year deal for £1.5m in 2009.
He made two substitute appearances in the Premier League and was an unused substitute in Chelsea's 1-0 win over Portsmouth in the FA Cup final in 2010.
Matic joined Vitesse Arnhem on loan for the season in August 2010 before moving to Portugal as part of the Luiz transfer in February 2011.
As well as Matic and Luiz, Chelsea also signed Brazil midfielder Ramires from Benfica in 2010 for about £17m.