"There are no words to express my sorrow and regret for the pain I have caused others by words and actions," Lauer said in a statement, read on air Thursday morning by his former co-host, Savannah Guthrie. "To people I have hurt, I am truly sorry."
Lauer's statement also said "some of what is being said about me is untrue or mischaracterized, but there is enough truth in these stories to make me feel embarrassed and ashamed."
"I regret that my shame is now shared by the people I cherish dearly," he said. "Repairing the damage will take a lot of time and soul searching, and I'm committed to beginning that effort. It is now my full-time job."
Lauer had been an anchor on the NBC morning program for two decades.
Guthrie and "Today" co-host Hoda Kotb announced the firing decision by NBC news chairman Andy Lack on air early Wednesday.
"This is a sad morning here at 'Today' and at NBC News," Guthrie said, before sharing a statement Lack issued to employees.
"On Monday night, we received a detailed complaint from a colleague about inappropriate sexual behavior in the workplace by Matt Lauer. It represented, after serious review, a clear violation of our company's standards. As a result, we've decided to terminate his employment," Lack wrote.
NBC confirmed the complaint stemmed from an incident during the 2014 Sochi Olympics, and Lauer's misconduct continued after the games.
In the statement, the chairman noted that this is the first official complaint about Lauer in more than two decades, but NBC was "also presented with reason to believe this may not have been an isolated incident."
"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," Lack concluded.
Shortly afterward, Variety reporter Ramin Setoodeh revealed over Twitter that he and reporter Elizabeth Wagmeister had been working on a story about "serious sexual harassment allegations" against Lauer involving "multiple victims."
Setoodeh said NBC was "aware" of the story, which he called a "bombshell."
Journalists at The New York Times were also reportedly investigating the "Today" host, per CNN's Brian Stelter.
Lauer's firing appeared to shock Guthrie. The co-host said she and her NBC colleagues "are devastated" and "still processing" the news.
"I will tell you right now, we do not know more than what I just shared with you," Guthrie said. "But we will be covering this story as reporters, as journalists. I'm sure we will be learning more details in the hours and days to come, and we promise we will share that with you."
She continued: "We are heartbroken. I'm heartbroken for Matt. He is my dear, dear friend and my partner, and he has been loved by many, many people here. And I'm heartbroken for the brave 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," Guthrie added. "But I do know 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."
Kotb, who had been called in last-minute to replace Lauer, said it had been a "tough morning."
"It's hard to reconcile with what we are hearing with the man who we know, who walks in this building every single day," she said.
During her portion of "Today," Megyn Kelly said Lauer "has been a friend."
"I see the anguish in my colleagues' faces," she said. "But what we don't see is the pain on the faces of those who found the courage to come forward, and it is a terrifying thing to do. We don't see the career opportunities women lose because of sexual harassment, the intense stress it causes a woman dealing with it when she comes to work each day."
"As painful as this moment is for so many here at NBC today — at CBS earlier this month, at Fox News over the past year, in Hollywood this fall — it is a sign of progress, of women finding their voices, their courage, and of the erosion of a shameful power imbalance that has been in place for far too long," Kelly added.