Mrs Obama was referring to the explosive video of Trump that leaked six days ago, plunging his campaign into turmoil and prompting dozens of Republican leaders to disendorse their own presidential nominee. In the tape, Trump described his attempts to sleep with a married woman and bragged he could "do anything" to women because he was a celebrity.
"When you're a star, they let you do it. Grab them by the p***y. You can do anything," he said, among a few other vulgarities.
In the days after the leak, which included his appearance at the second presidential debate, Trump apologised for his words, dismissing them as "locker room banter". Mrs Obama clearly didn't buy that explanation.
"This is not something that we can ignore. This is not something that we can sweep under the rug as just another disturbing footnote in a sad election season, because this was not just a lewd conversation. This wasn't just locker room banter," Mrs Obama said.
"This was a powerful individual speaking freely and openly about sexually predatory behaviour. And actually bragging about kissing and groping women using language so obscene that many of us worried about our children hearing it when we turned on the TV.
"And to make matters worse it now seems very clear that this isn't an isolated incident. It's one of countless examples of how he has treated women his whole life."
Speaking of those "countless examples", 10 different women publicly accused Trump of sexual misconduct yesterday:
• Natasha Stoynoff, a writer for People magazine, claimed Trump had "forced his tongue down her throat" and told her they were going to have an affair;
• Four former beauty pageant contestants told Buzzfeed Trump would walk into their changing rooms while they were getting dressed. They were teenagers at the time;
• Rachel Crooks told the New York Times Trump kissed her "directly on the mouth" without her permission;
• In the same story, Jessica Leeds claimed Trump touched her inappropriately on a flight, saying "his hands were everywhere";
• A former Miss USA contestant told the Guardian Trump barged in and "stared at" her and another woman as they were getting dressed;
• Tasha Dixon, another beauty contestant, told CBS walked into a changing room full of naked and half-naked women;
• Mindy McGillivray told the Palm Beach Post Trump groped her, with his wife Melania standing nearby.
Trump has denied all of those accusations, and specifically threatened to launch legal action against the New York Times. In a speech today, he seemed to imply Ms Stoynoff's claims weren't credible because she wasn't good-looking enough.
"Look at her. Look at her words. You tell me what you think. I don't think so," Trump said.
In her own speech, Mrs Obama said she took Trump's words and actions personally.
"I listen to all of this, and I feel it so personally. And I'm sure that many of you do too, particularly the women. The shameful comments about our bodies. The disrespect of our ambitions and intellect. The belief that you can do anything you want to a woman. It is cruel. It's frightening. And the truth is, it hurts. It hurts.
"To dismiss this as everyday locker room talk is an insult to decent men everywhere. The men that you and I know don't treat women this way. They are loving fathers who are sickened by the thought of their daughters being exposed to this kind of vicious language about women. They are husbands and brothers and sons who don't tolerate women being treated and demeaned and disrespected. And, like us, these men are worried about the impact this election is having on our boys who are looking for role models for what it means to be a man.
Of course, Mrs Obama's speech had a political purpose - to win votes for the nominee of her party, Hillary Clinton. Still, she insisted she wasn't just playing politics.
"This is not normal. This is not politics as usual. This is disgraceful. It is intolerable and it doesn't matter what party you belong to - Democrat, Republican, independent - no woman deserves to be treated this way. None of us deserve this kind of abuse.
"In our hearts, we all know that if we let Hillary's opponent win this election then we are sending a clear message to our kids that everything they're seeing and hearing is perfectly okay. We are validating it. We are endorsing it.
"I know it's a campaign, but this isn't about politics. It's about basic human decency. It's about right and wrong, and we simply cannot endure this or expose our children to this any longer. Not for another minute, let alone for four years. Now is the time for all of us to stand up and say enough is enough."
"Let's be very clear, strong men, strong men, men who are truly role models don't need to put down women to make themselves feel powerful."
