Trump's Twitter outburst -- in which he describes the journalist "bleeding badly from a facelift" -- appears to have been trigged by Brzezinski's sharp comments during the morning show she co-hosts with Joe Scarborough on the left-leaning MSNBC network.
"I heard poorly rated @Morning_Joe speaks badly of me (don't watch anymore)," wrote Trump, an avid cable news watcher.
"Then how come low I.Q. Crazy Mika, along with Psycho Joe, came to Mar-a-Lago 3 nights in a row around New Year's Eve, and insisted on joining me. She was bleeding badly from a face-lift. I said no!" Trump tweeted.
In recent days Trump has escalated his attacks on US media outlets -- which he accuses of peddling "fake news" about him -- singling out CNN over the retraction of a story about his administration's supposed links with Russia, but also assailing rival cable networks, the New York Times and the Washington Post.
But the vulgarity of his latest assault triggered a torrent of criticism, including from Republican Senator Lindsey Graham who called him out directly, tweeting: "Mr. President, your tweet was beneath the office and represents what is wrong with American politics, not the greatness of America."
Trump 'out of his mind'
Brzezinski herself appeared uncowed by Trump's onslaught. She responded with a post of her own, a photo of a young child reaching for Cheerios cereal.
"Made for little hands," the picture caption read -- a schoolyard taunt that has been leveled against the image-conscious Trump since the days of his candidacy.
In the Thursday show, Brzezinski had suggested the president was psychologically unhinged.
"Everybody in Washington, in the administration, needs to look at this from... a point of view where they're not lobotomized because you're sitting in there and you're so scared of him, and you think you need to suck up to the president."
If a boss at NBC, MSNBC's parent company, "started tweeting wildly about people's appearances, bullying people, talking about people in the competition, lying every day, undermining his managers, throwing them under the (bus) -- that person would be thrown out.
"It's just not normal behavior," she said. "In fact, there would be concerns that the person who runs the company is out of his mind."
Trump was often interviewed on "Morning Joe" during the 2016 campaign, but as the White House relationship with much of the US media has soured he has recently favored interviews with friendly right-wing outlets.
Brzezinski, 50, is the daughter of the late Zbigniew Brzezinski, who was president Jimmy Carter's national security advisor.
Scarborough, 54, is a Republican former congressman who represented a Florida district in the 1990s.
The two are engaged to marry.
Twitter ablaze
MSNBC immediately sprang to the defense of its host.
"It's a sad day for America when the president spends his time bullying, lying and spewing petty personal attacks instead of doing his job," the cable network's public relations office wrote.
Scarborough likewise rallied behind his fiancee, re-tweeting a string of posts assailing Trump, including one from NBC journalist Mark Kornblau that read: "Never imagined a day when I would think to myself, 'it is beneath my dignity to respond to the President of the United States.'"
White House spokeswoman Sarah Huckabee Sanders, speaking on Fox News, was quick to defend her boss.
"I don't think that the president's ever been someone who gets attacked and doesn't push back," Huckabee Sanders said.
"There have been outrageous number of personal attacks, not just to him but to frankly everyone around him ... This is a president who fights fire with fire and certainly will not be allowed to be bullied by liberal media or liberal elites in Hollywood or anywhere else."
But the Trump-Brzezinski feud lit up the internet, where critics were reminded of Trump's misogynistic attack last August on Fox reporter Megyn Kelly, who he said had "blood coming out of her eyes, blood coming out of her wherever" after she challenged him in a campaign debate.
"Harry Potter" author J.K. Rowling quoted a revered Trump predecessor in her reaction: "'Nearly all men can stand adversity, but if you want to test a man's character, give him power.' Abraham Lincoln."
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