Kylian will start his time with Chelsea as part of their development squad and the midfielder could make his debut in the understudies’ fixture against Sunderland on September 9.
“Kylian Hazard has today signed for Chelsea from Hungarian side Ujpest, and will join our development squad,” a statement on the club’s website announced.
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 Hungary in 2015 and made 42 appearances for Ujpest.
Kylian is the third Hazard brother to join Chelsea after Thorgan spent 2012 to 2015 at Stamford Bridge, although he was loaned out for much of that time.
Eden has yet to play for the Premier League champions this season after suffering an ankle injury at the end of last term, but he has joined up with Belgium this week for their World Cup qualifiers.