Lauer's co-host Savannah Guthrie announced his termination at the beginning of Wednesday's "Today" program.
"This is a sad morning here at 'Today' and at NBC News," Guthrie said.
Lauer, 59, is one of the best known faces on America's popular early morning news shows.
He thus joins the ranks of other titans of American entertainment, journalism and politics who have fallen amid allegations of sexual harassment, assault or rape in a groundswell of outrage that began with a flood of charges against movie mogul Harvey Weinstein.
In a memo to staff, NBC chairman Andrew Lack said that on Monday night the network received a detailed complaint about inappropriate sexual behavior in the workplace by 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.
Lack said this was the first complaint against Lauer in his more than 20 years with NBC but that the network was provided with evidence suggesting this may not have been an isolated incident.
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