The Oscar and Grammy winner and new mom says her life changed when she joined the weight-loss program.
"I decided to do Weight Watchers," she told Robin Roberts. "I feel like I've conquered the world."
In the seven months since her son David Jr. was born, the 28-year-old Hudson says she has taken off all the baby weight ... and more. She says everyone asks her how she slimmed down.
"No one recognizes me anymore," she said. "I wanted to go in a different direction and find a better lifestyle."
Hudson started the program right after giving birth, hoping to get into shape post-pregnancy.
In a new commercial for Weight Watchers, she says she's never been this size in her adult life. She wouldn't say just how many pounds she's dropped, but did say, "I've just reached a new milestone. It's about the progress and being comfortable."
In the past, Hudson has been vocal about being proud of her curves. After her Oscar-winning performance in "Dreamgirls" in 2007, she told ABC News that "[weight] is definitely a part of the industry, but it keeps you separated, and helps you stand out from other people. ...They need some normal-sized people around here, so that doesn't bother me at all."
Hudson says her recent weight loss isn't about fitting into a Hollywood mold of what size a woman should be.
"I was very comfortable with myself before," she told Roberts. "I have a child now and I want to set a great example for him. Make better life choices, health choices, and just create a good example for him growing up."
Weight Watchers has helped her balance working and parenting with a healthy lifestyle, she said, because she can eat what she wants as long as she stays within her plan's point limits.
"It's so much more convenient," she said.
On the days she doesn't make it to the gym, Hudson says just taking care of her son is a workout.
"It is about balance and finding what works for you," she said. "I put in my routine: let's get up and walk."
Hudson said she doesn't feel as if she's depriving herself. Her favorite foods include turkey wings and sweet potatoes -- and she knows how many points they are.
The change, she said, is all about making healthy choices and being aware of "what I'm putting in my body. Being more conscious of what I'm taking in."
Hudson's partner, David Otunga, is a Harvard Law School grad, but he's also an aspiring professional wrestler. He's already in good shape, but Hudson said even he is getting into the plan.
"He's been extremely supportive," she said. "Everybody around me … it's again about making better health choices."
Up next for Hudson is a trip to South Africa to play the role of Winnie Mandela, which she called "the opportunity of a lifetime."
"I'm so honored and extremely excited about it and very serious about it," she said. "It's an honor to play such a person."
But the best part of her life, she said, is being a mom.
"It is the best thing ever, I have the best baby ever and that makes it easier. He is an angel."