Last year, the annual event came under fire for turning away a group of women who weren't wearing heels, and the actress is making her views on the dress code rule very clear.
"Things have to change immediately," Stewart reportedly told Indiewire.
"It has become really obvious that if [a man and I] were walking the red carpet together and someone stopped me and said, ‘Excuse me, young lady, you're not wearing heels. You cannot come in.' Then [I'm going to say], ‘Neither is my friend. Does he have to wear heels?' It can work both ways.
"It's just like you simply cannot ask me to do something that you are not asking him," she continued. "I get the black-tie thing but you should be able to do either version—flats or heels."
Although the actress doesn't agree with the no-flats rule, it doesn't mean she's shunning heels altogether.
"They're beautiful," she said of her footwear choice on the first day of the festival (below). "I love them and they look amazing."
Julia Roberts also took a stand - quite literally - against the policy, walking the red carpet completely barefoot.