The 70-year-old billionaire took the stage in swing state Florida shortly after Obama finished her stump speech for Hillary Clinton, and decried the Democratic nominee as complicit in a media attempt to derail his campaign.
"This is a powerful individual speaking freely and openly about sexually predatory behaviour", she said.
"An at-times emotional Obama admitted that she "couldn't stop thinking about" the comments made by Trump and had to address them in her speech". "It has shaken to me to my core in a way that I couldn't have predicted", the first lady said. "This is not something we can sweep under the rug as just another disturbing footnote in a sad election season".
The writer says she didn't publicise the incident at the time because she was ashamed, afraid of the repercussions and blamed herself for what she says happened.
Obama said perhaps women are trying to hide how much this behavior bothers us because "we think that admitting how much it hurts makes us, as women, look weak".
But actress Arianne Zucker said she wasn't shocked by it, given "that type of personality". It risked amplifying Michelle Obama's central point: that Trump is a candidate who belittles real concerns women have about sexual assault.
He added: "I'm exhausted of new politicians who want to go to Washington to demean women". "It's one of countless examples of how he has treated women his whole life", she declared.
Clinton was to attend a fundraiser in San Francisco that includes a performance from singer Audra Day, with tickets costing as much as $50,000.
In the past days, the "Woman Can Stop Trump" movement has grown in influence, with many women bravely speaking out about the times when men like Trump asserted power over them in a way they did not consent to - sexually or otherwise - as Trump boasted in the Access Hollywood tape. The Republican nominee denied the story on Thursday, calling it "absolutely false".
Revealing that she felt compelled to speak out after Trump's 2005 hot mic moment was published by The Washington Post, Obama said, "This isn't about politics".
Obama also talked about the importance of women's votes: "Remember this: in 2012, women's votes were the difference between Barack winning and losing in key swing states, including in New Hampshire".
"Describing this as "locker room talk" is an insult to men everywhere", Obama asserted.
Clinton lauded the first lady's remarks.
By electing Trump, she said, "We are validating [his behavior]".
It could mean an improbable Utah victory for Democrat Hillary Clinton if she captures just one-third of the state's votes.