Fabregas scored 22 goals for the London club in 198 appearances.
The 31-year-old signed a three-and-a-half-year-deal with the French club.
"We thank Cesc for his wonderful service and wish him the very best of luck for the next chapter in what has been a fantastic career, ” said Chelsea in a statement. It is a quality squad with young players and a young coach", said Fabregas, referring Henry.
Having enjoyed a glittering career, capped by winning the World Cup with Spain in 2010, Fabregas arrives at a club who were French champions just 18 months ago but are now in crisis.
"Wow. I'm honored to announce that I will be starting an exciting journey with this wonderfully historic club".
"We are convinced that his experience at the highest level will bring a lot to our players on and off the field". Chelsea still went on to win the game 2-0, though, and Fabregas was given a standing ovation when he was substituted later in the game.
Current Chelsea manager Maurizio Sarri is another who changed Fabregas, asking him to become a regista and dictate play from deep areas.
The Premier League club are understood to be interested in deals for Cagliari midfielder Nicolo Barella and Zenit St Petersburg midfielder Leandro Paredes.
The Spanish midfielder began his career in England at Arsenal after joining them from Barcelona's academy and was named as the club's captain in 2008.