It's believed to have happened when they starred together in the 1998 film Twilight, which also featured Gene Hackman.
Sarandon told BBC 5 live that she discovered that her two male co-stars were getting paid the same as each other - and more than her.
She said Newman stepped in and said: "Well I'll give you part of mine."
Story, Sarandon told 5 live: "Emma Stone once came forward and said she got equal pay, because her male stars insisted upon it and gave up something of theirs.
"That happened to me with Paul Newman at one point, when I did a film with him ages ago."
She continued: "They said it was 'favoured nations', but they only meant the two guys.
"He stepped forward and said, 'Well I'll give you part of mine'. So, yeah, he was a gem."
The term "favoured nations" is an agreement between a producer and an actor, which ensures the actors are given the same terms as their co-stars.
In Twilight, all three stars were given equal billing on the poster, but Sarandon has now revealed that only Newman and Hackman benefited from the favoured nations arrangement.
On International Women's Day, Susan Sarandon also told 5 live that there will "always be a casting couch" in Hollywood.
"I think what will go away is the unwanted exchange.
"But I think that giving yourself sexually, or being drawn to power and wanting to have sex with someone that's in power, is also a choice.
"What we don't want to have is being exploited and have the Harvey Weinsteins of the world holding it over your head and holding it over your project.
"That is the most despicable."