The new deal will keep the 23-year-old at West Brom until the summer of 2014.
“I’m delighted Youssouf has signed his new contract, which is fully deserved,” West Brom boss Roberto Di Matteo told the club’s official website.
“He’s had a very good season so far and is a player who wants to learn.
“He’s a big favourite with the fans and I think that is down to the fact they recognise he has a specific role here.
“He does a little bit of the dirty work for the team and is a player who gives us balance. He also chips in with some goals, which aren’t expected but are a bonus.
“We’re all looking forward to carrying on working with Youssouf for years to come.”