The Academy said it made this decision following its new procedure for enforcing a Standards of Conduct that it adopted in the wake of the Harvey Weinstein scandal, which shocked the entertainment industry past year.
Presumably this standard didn't exist in 2003, when the Academy handed Polanski the Best Director Oscar despite him having pled guilty to unlawful sex with a minor in 1978. He faces up to 30 years in prison and his lawyer has vowed to appeal the conviction.
Cosby, meanwhile, was found guilty on three counts of aggravated indecent assault just last month, which followed years of allegations that he made a habit of giving women quaaludes before raping or assaulting them.
The film academy said Thursday that its board of governors met Tuesday night and voted on their status in accordance with their Standards of Conduct. His name has also been removed from the Academy list of Hall of Fame honorees online.
Polanski's latest film, Based on a True Story, premiered at the Cannes Film Festival last year, and was scheduled to be distributed in the US this year by Sony Pictures Classics.
Four Academy members are now known to have been expelled.
Over the past year, Hollywood has finally begun to crack down against abuses of power, particularly by rich men with long histories of allegations or confirmed crimes.
The film academy came under intense scrutiny following Weinstein's expulsion and the rise of the #MeToo movement for some of its active members, like Cosby, Polanski and Mel Gibson.
That will continue to make people feel uneasy - but this does show that the academy appears to be serious in its promise to tackle these issues and that the new procedures post-Weinstein are having an effect.
Representatives for Cosby and Polanski did not immediately return messages seeking comment.