The 29-year-old Brazil defender, who moved to Parc des Princes for £50million ahead of the 2014 World Cup, returns to Stamford Bridge for a reported £32million fee.
He said on chelseafc.com: “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.”
The Blues and PSG earlier on Wednesday announced terms had been agreed for Luiz’s return and the deal was announced soon after the 2300BST closure of the summer transfer window.
His acquisition was part of a busy deadline day for the Blues, who signed Marcos Alonso from Fiorentina and let Juan Cuadrado join Juventus on a three-year loan deal.
Luiz joined Chelsea from Benfica in January 2011 and made 143 appearances, scoring 12 goals, before his departure three-and-a-half-years later.
He won the 2012 Champions League and 2013 Europa League with Chelsea and the 2012 FA Cup.
He has played against Chelsea four times in the two years since leaving the Blues, as PSG were opponents in the Champions League in each of the last two seasons.
He vowed not to celebrate if he scored against his former club, but did so as PSG advanced on away goals at the expense of Chelsea in March 2015.
He is the latest former player to return to Chelsea. The Blues bought back Nemanja Matic, who joined Benfica as part of the deal which saw Luiz signed, while Didier Drogba had a second spell at Stamford Bridge.
Luiz made 89 appearances for PSG, scoring eight goals, and won two French Ligue 1 titles (2015, 2016), two Coupes de France (2015, 2016), two Coupes de la Ligue (2015, 2016) and two Trophees des Champions (2015, 2016).
He became the Blues’ fifth summer signing after France midfielder N’Golo Kante, Belgium striker Michy Batshuayi, Portuguese goalkeeper Eduardo and Alonso.