Her niece told the Associated Press that Douglas died in Baton Rouge, La., near her hometown of Zachary, and a spokeswoman for the East Baton Rouge Parish coroner's office also confirmed her death to the Los Angeles Times.
The cause of death was pancreatic cancer, Charlene Smith said. Her age was variously reported as 81 or 82.
Douglas played Elly May, the naive but lovable daughter of Jed Clampett, who lived to take care of her "critters" in the famed CBS sitcom, which ran from 1962 to 1971. "The Beverly Hillbillies" chronicled the fish-out-of-water tale of the Clampetts' move from the Ozarks to Beverly Hills after the family struck oil.
Douglas starred in one of the most iconic scenes in the science-fiction series "The Twilight Zone."
Douglas' longtime agent, Jacqueline Stander, described the actress as a "country angel."
"She was a sweetheart," Stander said. "She used to go around and talk at churches and stuff the last few years of her life, and just speak of God and love."
Douglas had been living in Louisiana for a number of years, and spent the last few years of her life making personal appearances in the area, Stander said.