MSNBC issued a statement that it had ended its contract with the controversial host, with no further explanation. Olbermann hosted the network's top-rated show, but his combative liberal opinions often made him a target of critics.
Olbermann did not explain why he was leaving.
"MSNBC thanks Keith for his integral role in MSNBC's success and we wish him well in his future endeavors," the network said.
A spokesman said Phil Griffin, MSNBC's president, would not comment on Olbermann's exit. Spokesman Jeremy Gaines would say only that the acquisition of NBC Universal by Comcast, which received regulatory approval this week, had nothing to do with the decision.
Olbermann wasfrom the network for two days in November for donating to three Democratic candidates, which violated NBC News' policy on political donations. Olbermann complained that he was being punished for mistakenly violating an inconsistently applied rule that he had known nothing about.
The host apologized to fans - but not to the network.
Olbermann, before leaving the show with a final signature toss of his script toward the camera, thanked his audience for sticking with him and read a James Thurber poem.
The host reminisced about his brief goodbye at ESPN's "Sports Center" that was cut short to accomodate a result from a tennis match and his time at MSNBC, thanking his fans, highlighting the $2 million in donations they sent to National Association of Free Clinics in honor of his dying father.
"This may be the only television where in the host was much more in awe of the audience than vice versa," said Olbermann during his sign-off. "You will always be in my heart for that, and for the donations to the Kranich family in Tennessee and these victims of governmental heartlessness in Arizona..."
He thanked a series of people, including the late Tim Russert, but pointedly not Griffin or NBC News President Steve Capus
Olbermann's prime-time show is the network's top-rated. His evolution from a humorous look at the day's headlines into a pointedly liberal show in the last half of George W. Bush's administration led MSNBC to largely shift the tone of the network in his direction, with the hirings or Rachel Maddow and Lawrence O'Donnell in primetime.
But Olbermann was known for a mercurial personality behind the scenes and he was almost fired last year for the political donations. He quit a prime-time show on MSNBC in the late 1990s, complaining that management was making him report too much on President Bill Clinton's impeachment scandal.
He was particularly critical of Fox News Channel and his direct competitor, Bill O'Reilly, frequently naming him his "Worst Person in the World" in a segment popular with his fans. Bosses at NBC had discussed trying to keep the tone of the vitriol down.
MSNBC announced that O'Donnell, who had frequently filled in for Olbermann before starting his own 10 p.m. show, will take over Olbermann's time slot starting Monday. "The Ed Show," with Ed Schultz, would move to 10 p.m. Cenk Uygur of the Web show "The Young Turks," will fill Schultz's vacated 6 p.m. time slot.
Olbermann had signed a new four-year contract with MSNBC two years ago. It's unclear what his plans are now.
He could give a boost to struggling CNN's prime-time lineup, but Olbermann would mean CNN would make an abrupt shift in its nonpartisan policy. It was not immediately known how quickly Olbermann could switch to another job if he wanted to.