Kidder, who's best known for her role as Lois Lane in Superman, died "peacefully in her sleep" on Sunday, her manager, Camilla Pines, tells ET.
Franzen-Davis Funeral Home and Crematory in Livingston, Montana, tells ET that Kidder died at her home. The cause of death is unknown. She was 69 years old.
Kidder starred alongside the late Christopher Reeve in 1978's Superman, and reprised the high-profile role in three more films. More big parts included roles in 1979's The Amityville Horror and 1975's The Great Waldo Pepper opposite Robert Redford.
She was also a theater actress, notably appearing in a 2002 production of The Vagina Monologues. The Canadian actress worked steadily throughout her life, including as recently as earlier this year, with projects still being in pre-production according to IMDB.
Kidder was also known for being outspoken about mental illness since making headlines in 1996 after she disappeared for four days and was found in a backyard in Glendale, California. Kidder later said she suffered a manic episode.
Kidder had been married and divorced three times. She is survived by her daughter, Maggie McGuane, whom she shared with her first husband, novelist Thomas McGuane.