EXCLUSIVE. Margot Kidder, the actress who played Lois Lane in “Superman,” died Sunday in Montana … TMZ has learned.

Margot was at her home when she passed away, according to the spokesperson at Franzen-Davis Funeral Home in Livingston, MT. The cause of death is unknown at this point.

Bruce Becker, the Park County Attorney, tells TMZ … an unknown person called police to report Margot was unconscious and not breathing.

Police responded and found her dead. The Livingston Police Chief says foul play is not considered a factor, but her death is now under investigation.

Margot starred opposite Christopher Reeve in 1978’s “Superman,” and also in the 3 sequels.

She continued acting right up until this year, taking small roles in TV shows and movies – but also had stage roles … including “The Vagina Monologues” on Broadway.

Margot had a very public battle with bipolar disorder for years.

Margot became an advocate for mental health after that incident.

She’d been married 3 times, although she’d been single since the ’80s. Margot is survived by a daughter.