Chelsea re-signed David Luiz from Paris Saint-Germain for a fee believed to be in the region of £30m.
Luiz passed his medical and completed his return to Stamford Bridge five minutes before the summer transfer window was due to close.
Luiz arrived in London at St Pancras train station ahead of his proposed move and eventually completed terms on a three-year contract.
He was in Paris on Tuesday and, when he heard PSG had rejected Chelsea's interest in him, he sought out both PSG's president Nasser Al-Khelaifi and their sporting director - his fellow countryman Leonardo - in an attempt to convince them to let him go.
Luiz said: "I'm delighted to return to Chelsea. We had a fantastic story during my first time at the club and I want to help the team and Antonio Conte achieve a lot of success.
"I always had a wonderful relationship with the fans and I am looking forward to wearing the blue shirt at Stamford Bridge once again."
PSG had previously turned down approaches from Chelsea for both Luiz and his Brazilian team-mate Marquinhos, telling them that neither defender was for sale.
But a Chelsea statement on Wednesday evening read: "Chelsea Football Club and Paris Saint-Germain have agreed terms for the transfer of David Luiz back to London."
The 29-year-old Brazilian left Stamford Bridge for France two years ago for £50m, which remains a world record fee for a defender.
Chelsea first purchased Luiz in January 2011 from Benfica for £21m.
In his three-year stay in west London, Luiz made 143 appearances for Chelsea in all competitions, scoring 12 goals.