The DR Congo international has put pen to paper on a deal which will keep him at The Hawthorns until at least the summer of 2014.
There is also the option of a further 12 months in the club's favour.
It is Mulumbu's third new contract in the past 13 months and is a reward for his outstanding form last season.
Mulumbu formed a solid partnership with Paul Scharner in the centre of the park and finished as Albion's second highest scorer with seven goals.
The former Paris St. Germain player was voted the club's player of the year and player's player of the year.
West Brom head coach Roy Hodgson said: "It is great that Youssouf has been rewarded for his efforts last season and that we have been able to persuade him that his future lies with West Bromwich Albion.
"We're looking forward to seeing him build on his success of being named player of the season - and trying to confirm it again this season."
The club's sporting and technical director, Dan Ashworth, said: "I'm delighted Youssouf has signed another new contract. I can remember when he first joined us, as a practically unknown from PSG two and a half years ago, and he has come on in leaps and bounds since then.
"As we have previously stated, we are aiming to strengthen our squad this summer - not dismantle it - and we believe Youssouf will be a key player for us again this season. I'm sure he will now get his head down and continue the improvement he has shown since joining us."
Source: PA
Source: PA