Also, former "Prairie Home Companion" host Garrison Keillor was cut loose by Minnesota Public Radio over an allegation of "inappropriate behavior." Keillor, 75, said he inadvertently put his hand on a woman's bare back in an attempt to console her.
NBC news chief Andrew Lack said in a memo to the staff that the network received a complaint about Lauer's behavior on Monday and determined it was a clear violation of company standards. NBC said the misconduct started when Lauer and a network employee were at the Sochi Olympics in 2014 and continued beyond that assignment.
Lack said it was the first complaint ever lodged against Lauer, but "we were also presented with reason to believe this may not have been an isolated incident."
VIDEO: Good Morning America's report on Matt Lauer
Lauer, one of the most-liked and highest-paid figures in the TV news industry, becomes the second morning host in a week to lose his job over sexual misconduct allegations. CBS News last week fired morning host Charlie Rose after several women who worked for him complained about his behavior.
Messages to Lauer and his agent were not immediately returned, and the network would not say whether he denied or admitted to any wrongdoing. He is married with three children.
Lauer, 59, has essentially been the king of television morning news since first being paired with Katie Couric on "Today" in 1997. For many years, "Today" was the unquestioned ratings leader, until it was eclipsed by ABC's "Good Morning America" following the ugly 2012 firing of Lauer's co-host Ann Curry. The show had stabilized in recent years with Lauer's pairing with Savannah Guthrie.
VIDEO: Robin Roberts reacts to Matt Lauer's firing
Lauer's "Where in the world is Matt Lauer?" segments were popular for years, and he was used by NBC for other events. He had been scheduled to host the network's coverage of the Rockefeller Center Christmas tree lighting ceremony Wednesday evening.
He joins a lengthening list of media figures felled by sexual misconduct accusations this year. Besides Rose, they include Lauer's NBC News colleague Mark Halperin, Fox News prime-time host Bill O'Reilly and National Public Radio newsroom chief Michael Oreskes. The New York Times suspended White House correspondent Glenn Thrush last week.
Ari Wilkenfeld, the lawyer for Lauer's accuser, praised NBC for acting "quickly and responsibly" in response to the morning host's "egregious acts of sexual harassment and misconduct." The attorney did not identify his client.
Lack, in his memo, said, "We are deeply saddened by this turn of events. But we will face it together as a news organization - and do it in as transparent a manner as we can."
As for Keillor, Minnesota Public Radio said it will end distribution of the radio program "The Writer's Almanac," Keillor's daily reading of a poem and telling of literary events, and end rebroadcasts of old "Prairie Home Companion" episodes.
"I meant to pat her back after she told me about her unhappiness and her shirt was open and my hand went up it about six inches. She recoiled. I apologized. I sent her an email of apology later and she replied that she had forgiven me and not to think about it," Keillor said in an email to the Minneapolis Star Tribune. "We were friends. We continued to be friendly right up until her lawyer called."
At NBC, Lauer's former "Today" colleagues, Guthrie and Hoda Kotb, announced the firing on the show Wednesday morning shortly after being told themselves.
Guthrie appeared to fight back tears as she called Lauer her friend who is beloved by many at NBC, and said she was "heartbroken for my colleague who came forward to tell her story and any other women who have their own stories to tell."
"How do you reconcile your love for someone with the revelation that they have behaved badly? I don't know the answer to that," she said. "But I do know that this reckoning that so many organizations have been going through is important, it's long overdue and it must result in workplaces where all women, all people, feel safe and respected."
Later in the show, NBC's Megyn Kelly, who has written about being sexually harassed by former boss Roger Ailes at Fox News Channel, noted the anguish on the faces of her colleagues but said, "What we don't see is the pain on the faces of those who found the courage to come forward, and it is still a terrifying thing to do."
Natalie Morales, a former "Today" show colleague who left the show for "Access Hollywood" last year, said she was in shock about the story. She referred to stories that said she and Lauer had an affair, which they both denied.
VIDEO: NY Times columnist reports on Lauer's firing
"I have personally dealt with rumors in the past few years that were hurtful to me and to my family," she said. "They diminished my hard work. I've addressed these rumors head-on in the past. It's not the story today."
On Twitter, President Donald Trump said, "Wow, Matt Lauer was just fired from NBC for 'inappropriate sexual behavior in the workplace.' But when will the top executives at NBC & Comcast be fired for putting out so much Fake News."
Another former "Today" host, Deborah Norville, said she was stunned by the news.
"As a journalist, it's upsetting to see another from our ranks caught up in these kinds of scandals. But as we've seen, no profession is immune," she said. "Sadly, I doubt if anyone will be surprised if there are similar headlines to come."
Send a breaking news alert
Report a correction or typo
Learn more about the 6abc apps