The attacking midfielder, the younger brother of Eden, signed for the Hungarian club last summer.
The 22-year-old scored four goals in 35 appearances and becomes the third Hazard to sign for Chelsea after older brothers Eden and Thorgan.
Chelsea announced the capture of Hazard today and revealed he will go straight into their development squad ahead of their next match on September 9th after the international break.
We have today signed Kylian Hazard, who joins up with our development squad…
— Chelsea FC (@ChelseaFC) August 29, 2017
The Blues are working hard on completing a number of late deals with just 48 hours left in the transfer window.
Antonio Conte’s men have agreed a £35m deal with Arsenal for Alex Oxlade-Chamberlain and are hopeful of a deal for Leicester City ace Danny Drinkwater.
The Italian’s spent around £145m so far this summer on Antonio Rudiger, Alvaro Morata and Tiemoue Bakayoko but still has significant money to spend after the departures of Nemanja Matic, John Terry and Nathan Ake.