The Grace and Frankie star appeared on Thursday's edition of The Ellen DeGeneres Show to discuss 9to5: The Story of a Movement, a documentary released earlier this month that she featured in.
Upon seeing her, Ellen exclaimed: "Wow. You know, I say this to you every time I see you, but you look fantastic. You look better than you've ever ever looked."
Watch: Jane Fonda, 83, stuns fans with youthful appearance
When Jane remarked that she was nearly 84, a baffled Ellen responded by saying: "84! Oh my god, I pray I look[ed] like that ten years ago!"
Fans were also wowed by Jane's youthfulness, with one YouTube commenter saying: "84? Jane Fonda is 84? She looks 20 years younger!"
Another fan joked: "OMG, I am 36 and she looks younger!"
Fans and Ellen were in shock at Jane's real age
Ellen proceeded to compliment the star's natural greying hair, which looked incredible in the lighting.
A passionate Jane replied that she was glad she had allowed her hair to naturally grey, declaring: "Enough already with so much time wasted, so much money spent, so many chemicals, I'm through with that!"
Speaking about the new documentary, which follows the 9to5 movement, an organisation set up in the 1970s to improve working conditions for women and families, she explained that it showed how "important organising is for today."
Jane makes sure to keep herself extremely fit
Jane also spoke on the show about receiving her second dose of the coronavirus vaccine. She implored anyone who could to get vaccinated to do so, explaining: "I'm lucky, I didn't have any reaction to it, it certainly didn't hurt!"
The actress explained that even though she'd gotten the second dose, she would still be following coronavirus restrictions, like wearing masks and socially distancing.
"Things don't go back to normal just because you've had the vaccine," she said. "You can still spread it or pick it up."
Read more HELLO! US stories here
Like this story? Sign up to our newsletter to get other stories like this delivered straight to your inbox.