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The 56-year-old talk show host revealed live on her programme on Monday that her mother Vernita Lee gave up a child shortly after giving birth because she couldn't afford to raise her.
Oprah only learnt about the existence of Patricia, now 47, in November.
Patricia was placed into foster care before being adopted at age seven.
Oprah hailed the revelation the "miracle of miracles" and said the news left her "speechless".
Describing their first meeting as a "beloved moment", she added: "It's not something I ever thought would happen to me."
Revealing family secrets
The TV star - who was living with her father Vernon Winfrey when Vernita had Patricia - made the decision to reveal her "bombshell family secret" herself because she feared it would be "exploited" if she didn't.
Patricia - whose surname was not revealed - was then brought out to the audience, where she revealed she has two adult children, the first of whom she had when she was 17.
Explaining how she found out Oprah was her half-sister, Patricia said she had initially seen Vernita discussing her three other children on a TV interview in 2007 and began to work out her place in family history after her son Andre went online and found the birthdates of his mother's siblings.
Patricia said: "We realised that Oprah could be my sister."
The single mother said she kept her connection to Oprah a secret because she didn't want to cause a media circus.
She added: "I did not want her hurt."
