The 22-year-old arrives from Hungarian outfit Ujpest and has previously spent time alongside his eldest brother at Lille between 2011 and 2012.
Kylian becomes the third Hazard brother to join Chelsea after Eden and older brother Thorgan, the latter of which went on to join Borussia Monchengladbach permanently in 2015 though the Blues retain a buy-back clause.
A club statement read: "Kylian Hazard has today signed for Chelsea from Hungarian side Ujpest, and will join our development squad.
"The 22-year-old, a younger brother of Eden, is an attacking midfielder who started his professional career in his native Belgium, first with White Star Brussels and then Zulte-Waregem. He joined Ujpest in the summer of 2015 and played 42 times for them.
"Chelsea’s development squad is next in action at home to Sunderland on Saturday 9 September."
A fourth Hazard brother, Ethan, currently plays for the youth academy of Belgian outfit Tubize where his three older brothers have all previously spent time.
