The Descendants actor will appear in a one-off version of the production on March 3 in Los Angeles to raise money for the American Foundation for Equal Rights.
8 was penned by Oscar-winning screenwriter Black about the struggle against California's controversial 'Proposition 8', which outlawed gay marriage in the state.
Clooney has said he hopes the charity performance will provide a boost to the efforts of gays and lesbians seeking to legally marry.
"It is astonishing that gay and lesbian Americans are still treated as second-class citizens," Clooney said in a statement. "I am confident that, very soon, the laws of this nation will reflect the basic truth that gay and lesbian people - like all human beings - are born equal in dignity and right."
It was reported this week that Clooney is planning to film a biopic on 1960s comedy team Tom Smothers and Dick Smothers.
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Watch the Descendants trailer below: