The 23-year-old’s new deal will keep him at The Hawthorns until the summer of 2013.
Mulumbu joined West Brom on a permanent basis from French side Paris St Germain last summer.
“Graham Dorrans signed a new contract three days ago and I’m delighted that another of our talented young players has committed his long-term future to the club,” West Brom manager Roberto Di Matteo told West Brom’s official website.
“Youssouf was one of about five players who played in the majority of our games last season and he played a major role in our success.
“He was only out of the team a short while when I played Gonzalo Jara in the middle.
“The great thing about Youssouf is that he’s a very humble guy who wants to learn.
“He’s also going to get better and better because he is still young.”
