In a statement to the Lisbon stock exchange, Benfica said that they will receive £20.8m for the sale of Matic, who is now set for his second spell at Stamford Bridge. After arriving from MSK Kosice in 2009, he made three substitute appearances in the 2009/10 season and left in January 2011 as part of the deal which brought David Luiz from Benfica to west London.
"I am very happy for this opportunity to be back at this club," the 25-year-old Matic told Chelsea's website on Wednesday. "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 manager Jose Mourinho, whose side are two points behind Premier League leaders Arsenal, 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," the Portuguese boss added. "I am sure he will become a very important member of the squad and help us to achieve our ambitions."
Matic made his name at Kosice before first being signed for Chelsea by then-manager Carlo Ancelotti. Matic made his Chelsea debut aged 21 during a 4-0 league victory against Wolverhampton Wanderers, and also featured in the title-clinching final-day 8-0 win over Wigan in 2010.
In August 2010, he went on a season-long loan to Vitesse Arnhem to gain more first-team experience and although the transfer to Benfica was agreed mid-season, he remained in the Netherlands for the rest of that term.
During the 2012/13 season Matic was named Portugal's player of the month on three occasions and played in all but one of Benfica's matches on their way to the UEFA Europa League final, where they were beaten 2-1 by Chelsea in Amsterdam.