Saying she has faith in her "Annie Hall" director, the 71-year-old actor shared an interview from 26 years ago in which he denied the allegations.
"Woody Allen is my friend and I continue to believe him. It might be of interest to take a look at the 60 Minute interview from 1992 and see what you think," Keaton wrote alongside the link of the interview.
Allen once again dismissed Farrow's claims, which have resurfaced in light of the #MeToo movement.
She reiterated her allegations in a 2014 New York Times op-ed and other articles. She has often criticised Hollywood actors for not taking her claims seriously.
Keaton's remarks come after Alec Baldwin compared Farrow to "To Kill A Mockingbird" character Mayella Ewell, who falsely accused an African-American man of rape.
While Hollywood personalities such as Rebecca Hall and Timothee Chalamet have pledged to donate their salaries from Allen's to charity, Greta Gerwig and Colin Firth have said they would never collaborate with the filmmaker again.
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