Hanks, 65, called Philadelphia and Forrest Gump "timely movies, at the time, that you might not be able to make now".
He won Oscars for both roles, the first in 1994 and the second in 1995, but he says the role he played as a gay man in Philadelphia could not be repeated today.
Speaking as he promoted the new Elvis Presley biopic, he said: "Let's address: 'Could a straight man do what I did in Philadelphia now?'
"No, and rightly so.
Tom Hanks in Cannes
Doug Peters
"The whole point of Philadelphia was 'don't be afraid'.
"One of the reasons people weren't afraid of that movie is that I was playing a gay man.
"We're beyond that now, and I don't think people would accept the inauthenticity of a straight guy playing a gay guy."
Hanks continued in his interview with the New York Times magazine: “It’s not a crime, it’s not boohoo, that someone would say we are going to demand more of a movie in the modern realm of authenticity.
"Do I sound like I’m preaching? I don’t mean to.”
But Hanks' comments sparked some backlash online, with many pointing out that Ewan McGregor recently won an Emmy for playing Halston and Bradley Cooper is shooting a biopic of Leonard Bernstein.
It comes after British actor Eddie Redmayne recently expressed regret at accepting the role of a trans woman in the film A Danish Woman.
He said it was a "mistake" and the role is one "he wouldn't take on now".
Tom Hanks won an Oscar for his role in Philadelphia
PA