The actress-turned-activist, currently promoting her E! docuseries as well as a new memoir called "Brave," bluntly gave her unflattering opinion of her former "Charmed" co-star while talking to ABC News on "Nightline" Thursday.
"I don't like her," McGowan said. "Because I think she's a lie."
While Milano is credited with reviving and popularizing the #MeToo hashtag, which encourages women to share their personal stories of sexual harassment and abuse, McGowan doesn't believe her to be fully committed solving the problem.
"I'm not looking at this from the outside," she said. "I know [Milano is] married to a CAA agent. Do the math. Who's behind Time's Up? CAA. Where do they meet? CAA. Who needs good PR? CAA. Who are part of the pimp problem? CAA."
In response, Milano told "Nightline" that she respects McGowan and she wanted to give a platform to those who have been silenced for so long.
"I am and always have been completely supportive of Rose and admire her bravery and speaking out about her experiences," Milano said. "My goal throughout the past few months with both MeToo and the TimesUp movement has been to use my platform to give others a voice so we can all work together to stamp out sexual harassment and sexual assault."
In a CNN interview on Wednesday, McGowan walked back harsh criticism of another actress, Meryl Streep.
McGowan called the Oscar winner (currently nominated once again for her performance "The Post") a hypocrite after she learned Streep would be participating in the #TimesUp initiative by wearing all black at the Golden Globes, despite previously calling Harvey Weinstein "God" in a 2012 acceptance speech at the Globes.
"Actresses, like Meryl Streep, who happily worked for The Pig Monster, are wearing black at the Golden Globes in a silent protest," McGowan wrote in a since deleted tweet. "YOUR SILENCE is THE problem. You'll accept a fake award breathlessly and affect no real change. I despise your hypocrisy."
Nevertheless, Streep kept it classy when CNN asked her in early January to comment on McGowan's tweet.
"I'm sure in many ways she wished I knew," she said. "What happened to Rose in unbearable and it sticks a knife in everyone's heart that this man was allowed to continue his -- the way he worked on people, over the bodies of women. He made a business over the bodies of women."
"I would thank her for that," McGowan told CNN Wednesday. "Thank you, Meryl Streep, for that."
"When I fired off that tweet, people think it's me raging -- it was me turned in and crying because I had to see, you see, her call him God [at the 2011 Oscars] so it's not that I'm sure I wish she knew. It's more that it was part of their propaganda because everyone knew he wasn't."
"If you didn't know specifically it was about sexual assault, he's not a good man," she added. "Never has been, never was. Not at all. When someone calls him God, it reinforces his power to him. So, again, I'm talking about the establishment and I praise her for what she said, and I thank her and it means a lot to me. I hope we all heal, I hope we all do."