The 21-year-old midfielder arrives from English Premier League side Chelsea FC and has put pen to paper on a season-long loan deal at the Borussia Park.
"Thorgan is a promising young talent who can play in different positions in attack," commented the Foals' sporting director Max Eberl. "We’re delighted that he has chosen us as the club where he can take the next step in his career."
Hazard, whose elder brother Eden also plays for Chelsea and is currently preparing for a FIFA World Cup semi-final against Argentina, spent the 2013/14 campaign on loan at Zulte Waregem in his homeland, scoring 14 goals in 29 league appearances, as well as two in ten UEFA Europa League games.
He has one senior cap at international level and was named player of the year in Belgium in 2013.