The 26-year old DR Congo international has signed a two-year deal with the option for a third year as reward for his brilliant form in recent seasons.
"This is good news for everyone connected with West Bromwich Albion," said manager Steve Clarke.
"Youssouf is a talented player and the fact he is willing to sign a contract extension shows we're doing the right thing as a club.
"Youssouf is an important player for the group - the number of games he's played in the past few seasons underlines this fact.
"The really pleasing thing is that Youssouf should now be coming into his best years as a footballer and will hopefully spend them here."