Black's new play was written in response to California's passage of Proposition 8, which banned same-sex marriage in the state.
Anthony Edwards, 30 Rock's Cheyenne Jackson and The Simpsons' Yeardley Smith have also signed for the reading, which will be held on September 19 at New York City's Eugene O'Neill Theatre.
Tony-Award winner Joe Mantello will direct the one-off performance.
All proceeds earned from 8's ticket sales will benefit same-sex marriage advocacy groups.
Freeman is reportedly in negotiations to play a magician in the upcoming heist film Now You See Me.
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Watch a trailer for Morgan Freeman's upcoming film The Dark Knight Rises below: