"The program is really delivering results for me and I look forward to long term success, staying motivated and helping others do the same," Hart said.
Hart, Nutrisystem said, has lost 20 pounds on the food delivery diet since giving birth to her third child in September.
"She mirrors our values and will resonate with our customers as authentic," Nutrisystem said, in a statement announcing the deal with Hart, 36. "Moving forward, you'll see us have an appropriate mix of inspiring celebrities and real life customers who have also enjoyed success."
Hart currently stars on ABC Family's "Melissa & Joey," but is still best known for her roles on teen classics "Clarissa Explains It All," and "Sabrina, the Teenage Witch." She has three sons with her husband, musician Mark Wilkerson.