The star, pregnant with her first child, is said to have landed a $3 million USD deal with the company.
An industry insider told a US newspaper: 'The deal is in place, and after having her baby, she will start with WeightWatchers to lose her baby weight.'
Simpson, 31, hopes to wed sports star fiance Eric Johnson soon after the birth next spring.
A representative for WeightWatchers was unavailable for comment today.
It comes after she told that she was so overwhelmed by emotion the first time she felt her baby move that she broke down in tears.
'I felt [the baby] move the other day for the first time and I had big crocodile tears,' she revealed during an appearance on the Rachael Ray Show. 'It was the coolest thing ever. It did a flip or something. I was like, 'There it went. It's there, it's really there.''