Kylian, an attacking midfielder, will be added to the Premier league champions' development squad and could make his debut in their fixture against Sunderland on Saturday week.
The 22-year-old began his professional career in his native Belgium, where he played for White Star Brussels and Zulte-Waregem.
He moved to the OTP Bank Liga in 2015 and made 42 appearances for Ujpest.
Kylian is the third Hazard brother to join Chelsea after Thorgan - the middle of the three - spent 2012 to 2015 at Stamford Bridge, although for virtually all of that time he was loaned out.
Source : PA
Source: PA