The 19-year old, whose brother Eden has recently signed for the Blues, has signed at Stamford Bridge
Thorgan joins his older brother Eden at Stamford Bridge after completing his move from the Ligue 2 outfit.
Thorgan plays in a similar position to his sibling, an attack minded midfielder.
Being two years younger than Eden, Thorgan Hazard is yet to earn a cap for Belgium.
However he is a Belgian youth international after representing his nation at Under-16, Under-17, Under-18 and Under-19 level and played at the 2011 European Under-19 Championship.
He will join Chelsea's Under-21 squad, managed by Dermot Drummy, to continue his pre-season training and a loan move is being considered to continue his development.