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Sara Blakely, who became the youngest female billionaire in the world through her Spanx fortune, is also the first to join Warren Buffett and Bill Gates' Giving Pledge.

Buffett and Gates helped launch the Giving Pledge in June 2010, asking the super-wealthy to donate the majority of their fortune to philanthropic causes.

The group announced on Tuesday that Blakely and eight other signatories had committed to giving half or more of their wealth to charity. The group now has 114 signatories from 10 countries and 23 states plus the District of Columbia.

Blakely, 42, is worth $1 billion and became the youngest, self-made female billionaire in the world last year, according to Forbes. Her Spanx shapewear now has standalone retail stores in the U.S. and remains a privately owned company.

Read more: Spanx Founder Reveals How to Build a Billion-Dollar Business

"After I appeared on the cover of Forbes, I got a call from Bill Gates' office, saying Bill Gates wanted to have dinner with me," she told Forbes. Blakely had a meal in Miami with Bill and Melinda Gates and several other couples, Forbes reported.

Blakely said she is passionate about supporting women's causes. The company has already donated $1 million to Oprah's Leadership Academy for girls in South Africa.

Watch the Nightline Interview: Spanx: One Woman's Success Story

"Since I was a little girl I have always known I would help women," she wrote in her pledge letter. "In my wildest dreams I never thought I would have started with their butts. As it turns out, that was a great place to start! At Spanx we say it's our goal to make the world a better place, one butt at a time. With this pledge my goal is to make the world a better place…one woman at a time."

While Blakely is the first female billionaire to join The Giving Pledge, she is not the youngest. That honor goes to Dustin Moskovitz, Facebook co-founder, who is 28 years old.

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