The 23-year-old will spend the campaign with the Brussels-based club, who currently sit in second place in the Proximus League.
Head coach Graham Potter said, “This loan move gives Soufyan the chance to play regular football at a good standard — which we couldn’t guarantee this season.
“He came to the club with an impressive record from his time in Holland, and the aim this season is for him to get back to that level in Belgium.
“He’s overcome a bad injury he suffered during his time on loan with Sparta Rotterdam, but since recovering he has found his opportunities limited to under-23 football.
“This move gives Soufyan the chance of playing regular senior football. It’s a good move for him and us, and we will keep a close eye on his progress in Belgium.”
Ahannach signed for Albion from Almere City, having just helped his former club reach the Dutch second-tier play-offs in 2017. That season he scored an impressive 18 goals and provided 15 assists in 42 appearances.
The winger made his first appearance for Almere City in 2012 and hit the back of the net 32 times in 97 appearances for the Dutch club.
After making nine league appearances for Albion's under-23 side during the 2017/18 campaign, Ahannach joined Sparta Rotterdam on loan for the remainder of the season — but his time there was cut short by a long-term injury after 12 appearances.
On his return from injury, in the second half of last season, Ahannach made a further seven appearances for Albion’s under-23s, finding the back of the net in a 2-1 win over Derby County.
His first involvement with the senior team came last Tuesday night in the Carabao Cup at Bristol Rovers, as an unused substitute in a 2-1 win at the Memorial Stadium.