Winfrey said Alley, who is 5-foot-8, has lost 75 pounds, after hitting a high of 220 pounds. Her weight gain was documented in various unflattering paparazzi photos.
To the sounds of the Commodores' "Brick House," Alley, 55, strutted onto the stage in a maroon bikini on Monday's show.
Wearing a dress for an interview, Alley said she never would have had the courage to wear a bikini onstage when she was 30, despite being slender.
"I think women — I don't think we ever feel like we're good enough. We don't feel like we're thin enough or pretty enough or smart enough or work hard enough. And we are good enough," she said.
"The bikini thing is neither here nor there, other than the fact, you know, I am 55 years old. So I thought — come on, we are all good enough. And we look good enough. And we are not our bodies."
She joked about the "stripper hose" she bought to flatter her legs — and how she had initially wanted to wear them up to her neck.
Alley said she's learned how to make smart choices when dining out. For exercise, she dances and works out with a personal trainer. She's also quit smoking.
"We can decide to change anything — at any age," Alley said. "What I hope for all women is they feel good about themselves, they look the way they want to look."