The AA Gent attacker, who can play either as a striker or on either wing, will be assessed at the end of the 2012-13 campaign, with a permanent deal on the cards if both parties are happy.
Baggies boss Steve Clarke is glad to have strengthened his attacking options, and feels the 22-year-old can excel in the Premier League.
“I’m delighted to welcome Yassine to the club. He is an exciting, young prospect who will hopefully add something different to our squad,” he told the Midlands club’s official website.
“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.”
El Ghanassy has been capped twice by Belgium and scored seven goals in 37 appearances for Gent last term.
By Gareth McKnight