"Superman" actress Margot Kidder died from an intentional drug and alcohol overdose ... according to the Coroner's Office.

The Park County Coroner says Kidder's overdose was self-inflicted. TMZ broke the story ... Kidder, who played Lois Lane in the '70s and '80s "Superman" movies, was found May 13 in her Livingston, MT home. The Coroner has not specified what drugs she took.

Her daughter, Maggie McGuane, told the AP the family wanted to be "open and honest" about the suicide to avoid any "cloud of shame."