Alley, 57, told People that while her experience with Jenny Craig was "extraordinary," she wants "to create something new that will help millions of people end the seemingly never ending fatty-roller coaster ride."
Alley said she intends to develop her own weight-loss brand with plans to launch in 2009.
"We are sorry that she did not accept our offer" to continue appearing in the company's ad campaign, Scott Parker, vice president of marketing for Jenny Craig, told People. "We wish her all the best in her future endeavors."
A message left with Alley's publicist late Tuesday was not immediately returned.
Alley, who starred on Showtime's reality-comedy series "Fat Actress," became a spokeswoman for the Jenny Craig diet program in 2005. About a year later, Alley donned a bikini to show off her new shape on "The Oprah Winfrey Show." At the time, Winfrey said Alley, who is 5-foot-8, had lost 75 pounds, after hitting a high of 220 pounds.
Alley's TV credits also include "Cheers" and "Veronica's Closet."
