The Belgium international, who turns 22 on Thursday, could move to the Hawthorns permanently after his loan spell ends next year
The 21-year-old Belgium international, who turns 22 on Thursday, will move to the Hawthorns subject to international clearance, while West Brom have the option to make the loan spell permanent at the end of the season.
El Ghanassy scored seven times in 37 appearances for Gent last season, who finished third in Belgium's Jupiter Pro League.
"I'm delighted to welcome Yassine to the club," said West Brom boss Steve Clarke.
"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."