The AA Gent winger has joined on a season-long loan deal - subject to international clearance. The Baggies have an option to make the move permanent.
The 21 year old won the first of his two full international caps against Finland in February 2011.
Head coach Steve Clarke said: "I'm delighted to welcome Yassine to the club.
"He is an exciting, young prospect who will hopefully add something different to our squad.
"This is a big move for Yassine and we'll have to be patient with him as he settles into a new country and gets to grips with the demands of the Barclays Premier League.
"But we believe he has got the raw talent to make an impact here."