The Baggies have made the 22-year-old their second summer signing – subject to international clearance – with the option to make the move permanent.
El Ghanassy made for Jupiler League club Gent his debut in 2008, since when he has made 90 league appearances in Belgium and made the first of his two senior international appearances to date earlier this year.
West Brom 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."
El Ghanassy is the second incoming player to The Hawthorns this summer, following goalkeeper Ben Foster making his loan move from Birmingham City a permanent one.