The 87-year-old actress was living "out in the wild" in New Mexico and looking after her daughter Vanessa Vadim's son Malcolm, now 25, when she found the creature looming over him in his crib so took swift action to scare the animal away.
The story emerged when Fonda appeared on Netflix's podcast Skip Intro with her son Troy O'Donovan Garity and the 51-year-old actor was asked to share something people didn't know about his famous mum.
Garity - whose dad is Fonda's late ex-husband Tom Hayden - told host Krista Smith: "She pushed a bear out of her bedroom.
"She heard something so she left the bedroom in the middle of the night to go see what the sound was. And when she came back in, the screen door was dismantled and there was a bear in the bedroom, sniffing over the crib."
The host then asked the Grace and Frankie star how she reacted, and Fonda quickly held up her hands and shouted: "Roar!"
Garity jokingly apologised to the "sound man" on the podcast for his mother's loudness and the 80 For Brady actress explained the timing was lucky.
She said: "I had just learned what to do if you're close to a bear.
"And that was it (scream). And get very big; I opened my bathrobe."
While Fonda successfully scared away the bear, the creature "urinated on the rug" before it left.
She added: "And then (it) walked back to the door and sat down and I did, I pushed it out, yeah."
The Barbarella actress recently admitted she doesn't like shopping.
She told the New York Times newspaper: "I basically don't go shopping.
"Occasionally I will order something online if I'm on a trip and going into forests and need quick-drying pants and shirts - that kind of thing. But I don't like to shop. I feel uncomfortable."
Fonda previously bemoaned the rise of "consumerism" during an appearance on The Ellen DeGeneres Show.
The veteran movie star - who is also well-known for her environmental activism - admitted to feeling frustrated by the attitude of some shoppers.
She said: "I vowed a couple of years ago I would never buy any new clothes again.
"We spend too much money, we buy too many things, and then we get rid of them. We try to develop our identity by shopping, right? We gotta stop that. Stop all this consumerism."
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