El Ghanassy, who turns 22 on Thursday, has put pen to paper on a season-long deal which is with an option to make the move permanent.
He will link-up with West Brom once he has gained international clearance and Baggies chief Steve Clarke was delighted to have sealed his signature.
"I'm delighted to welcome Yassine to the club," he said.
"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."
Sporting and technical director Dan Ashworth confirmed that the club had been tracking the player for a while.
"Yassine is a player we have watched develop for a couple of years.
"He has shown in Belgium his ability to put defenders on the back foot and create and score goals and he is on the fringes of the national team.
"He is an exciting, young player who we hope will grow into the league and prove a real asset.
"He offers us a different option and is the type of player I am sure our supporters will warm to and get right behind."
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