The media mogul, 60, shared the news at the coffee giant's annual meeting in Seattle on Wednesday.
Walking onto stage in front of hundreds of shareholders, Oprah was given a warm welcome by Starbucks CEO Howard Schultz. The pair looked excited to make the announcement of their new partnership, and embraced in a big hug.
"Now we've known each other for a while," began Howard to the sounds of cheers and applause. "But last summer we were at a friend's wedding and Oprah was drinking tea. I didn't think much of the brand she was drinking."
"Yes," said Oprah. "Howard said, 'I have a tea for you, have I got a tea for you.'"
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The duo then explained that — as Oprah already makes her own blends — they came up with the idea of the television personality making her own signature blend for Starbucks.
"We thought, 'could we do something new?'" said Oprah, before yelling out to the audience, "Oprah Chai!" The audience broke into applause as she added, "At a Teavana near you."
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Oprah and Howard had paid a secret visit to the Starbucks headquarters to come up with the blend, which is described as a bold infusion of cinnamon, ginger, cardamom and cloves, blended with loose-leaf black tea and rooibos.
"I don't just like tea, I love tea," said Oprah. "And it really is a blend that I came up with, with the teaologists at Starbucks and Teavana."
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On stage, Oprah and Howard toasted to their new venture by clinking mugs of hot chai together.
"I've been up since 3:14 this morning," said a clearly excited Oprah, before the pair sat down to talk about their partnership in more detail.
The Teavana Oprah Chai Tea will be sold in Starbucks and Teavana stores around the US and Canada from 29 April. Parts of the proceeds will be donated to the Oprah Winfrey Leadership Academy Foundation to benefit educational opportunities for young people.
The philanthropist explained that she'd previously turned down opportunities to promote everything from cars to cosmetics to clothing.
"None of that felt organic," she said. "Starbucks is about nurturing the human spirit. That is my goal in life and what my brand has become."
Last year, the coffee chain opened the first Teavana Fine Teas + Tea Bar in Manhattan and has since expanded to Los Angeles, Chicago and Seattle. Starbucks hopes to open at least 1,000 more of its own Teavana bars in the US and abroad.