The actors began dating in 1988 after meeting on the set of Bull Durham.
They never married but went on to have two children -- sons Jack Henry, 20, and Miles Guthrie, 17.
Confirming the news to People.com, her representative Teal Cannaday says, "Actress Susan Sarandon and her partner of 23 years, actor Tim Robbins, have announced that they separated over the summer.
"No further comments will be made."
(c) 2009 WORLD ENTERTAINMENT NEWS NETWORK LTD. All global rights reserved. No unauthorized copying or re-distributing permitted.