The Golden Globe winner admitted Monday night on The Late Show with Stephen Colbert that she was offered the chance to play Eowyn in the popular trilogy, but passed it up.
"I do consider it one of the worst decisions ever made," the 46-year-old actress admitted. "But I just had my first child. I don't know, I was a little housebound."
Thurman gave birth to Maya, her daughter with ex Ethan Hawke, on July 8, 1998.
The role of Eowyn, one of the only female characters in the film franchise, eventually went to Miranda Otto, who went on to appear in War of the Worlds, and currently stars in 24: Legacy.
Colbert joked that as a hardcore J.R.R. Tolkein fan, even he would have taken the role if it was offered to him.
"It's definitely a regret," Thurman said.
Thurman visited the CBS late-night show to promote her new Bravo series, Imposters. On the show, she plays Lenny Cohen, an enforcer employed by a mysterious con woman.
Imposters airs Tuesdays at 10 p.m. ET/PT on Bravo.