The blonde bombshell, whose weight has been scrutinized in the last few years, reportedly fielded several offers from a number of other weight loss companies.
A source told the Post, "The deal is in place, and after having her baby, she'll start with Weight Watchers to lose her baby weight."
Rumors about the contract began last month, with reports saying that the company wanted to help Simpson, 31, get back to her famous "Dukes of Hazzard" physique, but the star reportedly wanted to sign on only to shed her baby weight.
According to a report from Us magazine last month, the company wants her to use their famous points system to follow in the footsteps of their other celebrity spokeswoman Jennifer Hudson to "lose a significant amount of weight."
Simspon plans to wed her fiance Eric Johnson after having the baby, which could be another motivation for the star to drop weight.