The fast-moving national reckoning over sexual harassment in the workplace toppled another television news star on Wednesday when NBC fired Matt Lauer, the co-host of its most profitable franchise, “Today,” after an allegation of inappropriate sexual behavior with a subordinate.

The downfall of Mr. Lauer, a presence in American living rooms for more than 20 years, adds to a head-spinning string of prominent firings over sexual harassment and abuse allegations. NBC News said it had decided to dismiss its star morning anchor after a woman met with network executives on Monday to describe her interactions with him.

“While it is the first complaint about his behavior in the over 20 years he’s been at NBC News, we were also presented with reason to believe this may not have been an isolated incident,” Andrew Lack, the NBC News chairman, wrote in a memo to staff.

On Wednesday, NBC received at least two more complaints related to Mr. Lauer, according to a person briefed on the network’s handling of the matter. One complaint came from a former employee who said Mr. Lauer had summoned her to his office in 2001, locked the door and sexually assaulted her. She provided her account to The New York Times but declined to let her name be used.