Chelsea unite Hazard with younger brother
Thorgan, 19, is two years younger than Belgian international Eden, who signed for Chelsea from Lille last month, and can play in a similar range of attacking midfield positions.
He joined Lens aged 14 and moved up to the senior squad last season, making 14 Ligue 1 appearances including his debut as a substitute in the first match of the campaign.
Thorgan has played for Belgium at several age groups and was part of the squad for the 2011 European Under-19 Championship.
He will join Chelsea's Under-21 squad for pre-season training with a loan move being considered so he can continue his development, the club added on their website.
The Hazards, whose parents were footballers, were both at Belgian team Tubize and could join a long list of brothers to play for Chelsea including Ray and Graham Wilkins, Ron and Allan Harris, John and Peter Sillett and Chris and William Ferguson