“Chelsea have completed the signing of Thorgan Hazard from FC Lens,” the champions of Europe said on their website.
“He will now join our Under-21 squad, managed by Dermot Drummy, to continue his pre-season training and a loan move is being considered to continue his development.”
Hazard, 19, links up with his older brother Eden Hazard, who arrived at Stamford Bridge from Lille in a £32 million deal earlier this summer.
The younger Hazard has only played 14 matches for French side Lens, but has 10 goals in 23 matches for Belgium’s U19 team.
He can play as a winger or an attacking midfielder and is the third signing this season, with Germany midfielder Marko Marin also moving from Werder Bremen.
Chelsea also confirmed that several youth-team players have left the club on loan for next season.
Forward Ulises Davila has joined Segunda Division side Sabadell, while striker Milan Lakovic and goalkeeper Matej Delac will play for Guimaraes in Portugal next term.