Jennifer Lawrence and America Ferrera also shared harrowing abuse stories, and Game of Thrones star Lena Headey revealed her troubling encounters with Harvey Weinstein.
As Hollywood reels from the Weinstein scandal, Witherspoon opened up about her career in a powerful speech reflecting on the problems faced by women in the film industry.
"This has been a really hard week for women in Hollywood, for women all over the world, and a lot of situations and a lot of industries are forced to remember and relive a lot of ugly truths," she said at an Elle Women in Hollywood event on Monday.
She continued: "I have my own experiences that have come back to me very vividly and I find it really hard to sleep, hard to think, hard to communicate a lot of the feelings that I've been having about anxiety, honest, the guilt for not speaking up earlier.
"[I feel] true disgust at the director who assaulted me when I was 16 years old and anger at the agents and the producers who made me feel that silence was a condition of my employment. And I wish that I could tell you that was an isolated incident in my career, but sadly it wasn't. I've had multiple experiences of harassment and sexual assault and I don't speak about them very often.
"But after hearing all the stories these past few days and hearing these brave women speak up tonight about things that we're kind of told to sweep under the rug and not to talk about, it's made me want to speak up and speak up loudly because I actually felt less alone this week than I have ever felt in my entire career."
She added: "I have just spoken to so many actresses and writers, particularly women, who have had similar experiences and many of them have bravely gone public with their stories. That truth is very encouraging to me and everyone out there in the world because you can only heal by telling the truth.
"I feel really, really encouraged that there will be a new normal. For the young women in this room, life is going to be different because we're with you, we have your back and it makes me feel better. It makes me so sad to talk about these issues, but I would be remiss not to."
The Big Little Lies producer's comments come as dozens of women accused disgraced movie titan Weinstein of sexual misconduct, including Ashley Judd, Rose McGowan, Asia Argento, Angelina Jolie and Gwyneth Paltrow.
In response to the lengthy allegations made against Weinstein, his spokesman said: "Any allegations of non-consensual sex are unequivocally denied by Mr Weinstein."
Weinstein also said in a statement he was working with therapists and planned to "deal with this issue head-on."
JENNIFER LAWRENCE ON HOLLYWOOD ABUSE
Actress Jennifer Lawrence has revealed a female producer forced her to do a nude line-up with five women and told her to lose weight.
The Mother star shared her story at Elle's Women in Hollywood event, where she took the podium to accept the magazine's cover honour.
"When I was much younger and starting out, I was told by producers of a film to lose 15 pounds (6.8kg) in two weeks," she said, revealing another actress before her had already been fired for not losing weight fast enough.
The 27-year-old continued: "During this time a female producer had me do a nude line-up with about five women who were much, much, thinner than me. We are stood side-by-side with only tape on covering our privates.
"After that degrading and humiliating line-up, the female producer told me I should use the naked photos of myself as inspiration for my diet."
Lawrence said she told another producer she thought the weight loss demands were inappropriate.
"He said he didn't know why everyone thought I was so fat, he thought I was 'perfectly f***able.'"
She added: "I let myself be treated a certain way because I felt I had to for my career."
AMERICA FERRERA REVEALS ASSAULT
Ugly Betty actress America Ferrera has revealed she was sexually assaulted at age nine.
The 33-year-old shared the harrowing details with the #MeToo hashtag, a movement sparked by actress Alyssa Milano who encouraged women on social media to speak out about their own experiences of harassment.
Ferrera's allegation comes as Hollywood reckons with a culture of abuse in the wake of the Weinstein scandal.
"First time I can remember being sexually assaulted I was 9 years old. I told no one and lived with the shame and guilt thinking all along that I, a 9 year old child, was somehow responsible for the actions of a grown man," Ferrera wrote on Instagram.
"I had to see this man on a daily basis for years to come. He would smile at me and wave, and I would hurry past him, my blood running cold, my guts carrying the burden of what only he & I knew - that he expected me to shut my mouth and smile back.
"Ladies, let's break the silence to the next generation of girls won't have to live with this bulls**t."
LENA HEADEY'S ALLEGED WEINSTEIN ENCOUNTERS
British actress Lena Headey has recounted two unsettling experiences she allegedly had with Weinstein.
Headey, who plays Cersei Lannister in HBO series Game of Thrones, wrote on Twitter that the first was in 2005 at the Venice Film Festival when the producer "made some suggestive comment and gesture" after asking her to walk with him by the water.
"I just laughed it off, I was genuinely shocked, I remember thinking, it's got to be a bad joke," she wrote on Tuesday.
The second, she claimed, happened years later in LA when Weinstein allegedly invited her to his hotel room to show her a script.
Headey wrote that Weinstein was "furious" when she told him she wasn't "interested in anything other than work".
She wrote that Weinstein whispered in her ear: "Don't tell anyone about this, not your manager, not your agent."
"I got in the car and I cried," Headey wrote.
BIG BANG STAR BACKTRACKS
Big Bang Theory star Mayim Bialik has apologised amid widespread backlash to her tone-deaf op-ed on the Weinstein scandal.
The 41-year-old's piece implied she hadn't been sexually assaulted or harassed because of how she looks.
Celebrities such as Patricia Arquette, Emily Ratajkowski and Gabrielle Union joined the chorus of outrage, accusing Bialik of blaming the victims of sexual assault.
"I still make choices every day as a 41-year-old actress that I think of as self-protecting and wise. I have decided that my sexual self is best reserved for private situations with those I am most intimate with. I dress modestly. I don't act flirtatiously with men as a policy," she wrote.
The actress, who "entered the Hollywood machine" in 1986 and played the title character on TV show Blossom from 1991-95, tried to clarify her statements on Monday.
"I have stayed off social media, but it has become clear to me that there are people who think that I either implied or overtly stated that you can be protected from assault because of the clothing that you wear or the behaviour that you exhibit," she said on Facebook Live.
"That is absolutely not what my intention was, and I think that it is safe for me to start this conversation by saying there is no way to avoid being the victim of assault by what you wear or the way you behave."
She continued: "I really do regret that this became what it became, because literally I was trying to speak about a very specific experience I had in a very specific industry - I was not looking to speak about assault and rape in general.
"I'm a human being, and there's a lot that I've chosen not to share, but absolutely I am deeply, deeply hurt if any women who has been assaulted - or man - thinks that in any way I was victim-blaming."
WEINSTEIN RESIGNS FROM BOARD
Weinstein on Tuesday resigned from the Weinstein Company board of directors.
Though he was fired as co-chairman on October 8, he continues to own 22 per cent of the company's stock, and until Tuesday morning continued to hold a seat on the board.
The company now risks falling into a protracted legal battle with Weinstein, who alleges his firing was illegal.
There also appears be tension between the remaining board members, who are exploring a sale to Colony Capital, and co-founder Bob Weinstein, who said in a statement last week that the company is not for sale.