The 31-year-old singer-actress confirmed the deal on Twitter and said she is excited about becoming a Weight Watchers ambassador, reported Femalefirst.
"So excited to be part of the Weight Watchers family," she posted.
Simpson gave birth to her first child named Maxwell Drew with fiancee Eric Johnson, 32. The actress is said to be keen to gain her pre-pregnancy figure with the new plan.
"The cool thing about the programme is that it focuses on healthy habits for the long-term (and I can still indulge in my guilty pleasures every now and then, too). I have actually gotten a group of friends together who are going to be doing it with me."
Actress Jennifer Hudson is the current spokesperson for Weight Watchers.