The 26-year-old DR Congo international has become a key player for the Baggies since his arrival from Paris St Germain in January 2009, and has been rewarded with an improved deal which will keep him at the Hawthorns until 2016 with the option of a further 12 months.
Mulumbu had a year to run on his previous contract and head coach Steve Clarke welcomed the player's decision to extend his stay.
"This is good news for everyone connected with West Bromwich Albion," he told the club's official website.
"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."