The Serbian midfielder left Stamford Bridge as a makeweight in a deal for David Luiz at the start of 2011 but has indicated he wants returns to the club.
The Serb, who has developed into one of Europe's finest holding midfielders since his transfer to Portugal in 2011, is expected to return to Stamford Bridge for a fee believed to be in the region of £20m.
The deal represents a reported £17m profit for Benfica, with the Blues re-signing a player they sold for around £3m sure to provide some form of financial embarrassment.
That fee, however, is offset by the fact Chelsea will recoup more than double their initial outlay on Kevin de Bruyne when he completes an expected move to Bundesliga side Wolfsburg, despite only making a handful of first-team run-outs for the Blues.
Matic, meanwhile, will add strength and discipline to a Chelsea midfield over-stocked with attacking midfielders.
"Yes, it was my last game," the 26-year-old told Portuguese publication A Bola when quizzed on his future.
"I want to thank all the people at the club and all the supporters. Benfica will be forever in my heart. I played here two and a half years, I gave my best in every practice, every game, and now is the right time to leave.
"I want to improve, I want to go to a better league and want to go to Chelsea, the club which I also like.
"I have always given my best for the team, the club and the supporters. I hope you recognize that and understand my position and my desire."
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