The 47-year-old Canadian-born actress, best known for playing Lois Lane in the Superman movies, was "in obvious mental distress" when officers found her on Tuesday in the Los Angeles suburb of Glendale, police said.
Kidder, missing since Saturday, reportedly said she had been assaulted and was hiding from someone, but police said they found no evidence she was the victim of a crime.
"At the time of her discovery, she was wearing dishevelled, cast-off clothing and apparently cut off her own hair with a razor blade in an attempt to alter her appearance," a Glendale police sergeant, Rick Young, said.
Kidder had bruises and scratches, but Young said her injuries were from living in the bushes for several days.
She was taken to Olive View Medical Centre for a 72-hour psychiatric evaluation, officials said. Family members expressed relief that she had been found alive and were heading for Los Angeles to see her.
Investigators remained puzzled over her bizarre behaviour but ruled out drugs or alcohol as the cause.
Kidder, whose career had been in decline since her Superman days, had battled against health problems in recent years after being seriously injured in 1990 during the filming of a TV series.
Her manager, John Blake, said she had been scheduled to fly from Los Angeles to Arizona on Saturday to teach a class, but when she failed to show up at the airport he filed a missing person report.
Glendale police responded to a call on Tuesday about a woman hiding behind a home on a fashionable street. Officers found Kidder there and described her as "dirty, frightened and paranoid".
Residents at first thought she was a homeless person because she did not live in the neighbourhood. Witnesses said it appeared her two front teeth had been knocked out but police denied it, saying her dental bridge was missing.
"At this time we are concerned for her well-being and mental health," Glendale police chief, James Anthony, said.