"MSNBC and Keith Olbermann have ended our contract," NBC Universal said in a statement Friday. "The last broadcast of 'Countdown' will be this evening. MSNBC thanks Keith for his integral role in MSNBC's success and we wish him well in his future endeavors."
The reasons for the liberal host's departure were not immediately clear, though he has had public difficulties with the network, including a suspension in November over donations he made to Democratic candidates in violation of network policy.
MSNBC spokesman Jeremy Gaines said the ongoing acquisition of NBC Universal by Comcast had nothing to do with the decision to let Olbermann go, according to the MSNBC website.
MSNBC anchor Rachel Maddow spoke about Olbermann's departure on Friday night's edition of "Real Time with Bill Maher."
"All I know is that it was between Keith and the company, it didn't involve any of the rest of us, and he was really gracious and nice when he left," she said.
Olbermann seemed to suggest before signing off Friday that he learned abruptly that his show was ending.
"I think the same fantasy has popped into the head of everybody in my business who has ever been told what I've been told -- that this is going to be the last edition of your show," he said. "You go directly to the scene from the movie 'Network,' complete with the pajamas and the raincoat and you go off on an existential, otherworldly verbal journey of unutterable profundity and vision. You damn the impediments and you insist upon the insurrections, and then you emit Peter Finch's guttural, resonant, 'So--.' And you implore, you will the viewer to go to the window, open it, stick out his head and yell-- Well, you know the rest.
"In the mundane world of television goodbyes, reality is laughably uncooperative," he added.
Rather than lashing out on the air, "Network"-style, Olbermann thanked his colleagues and viewers, and recalled an even more abrupt ending to his tenure at the ESPN show "SportsCenter."
"As God is my witness, in the commercial break just before the emotional moment, the producer got into my earpiece and he said, 'Um, can you cut it down to 15 seconds so we can get in this tennis result from Stuttgart?'" he said. "So I'm grateful that I have a little bit more time to sign off here."
In thanking "my gifted staff here and just a few of the many people here who fought with me and for me," he mentioned several colleagues by name but omitted NBC News President Steve Capus and MSNBC President Phil Griffin.
As he returned to the air after his suspension in November, Olbermann publicly thanked his viewers -- but not MSNBC.
Before getting to one last reading from author James Thurber, a Friday tradition on his show, Olbermann once again thanked his viewers for standing by him.
"The program grew and grew thanks entirely to your support," he said. "There were many occasions, particularly in the last two and half years, where all that surrounded the show, but never the show itself, was just too much for me. But your support and loyalty, and if I may use the word, insistence, ultimately required that I keep going. My gratitude to you is boundless. ... This may be the only television program wherein the host was much more in awe of the audience than vice versa."
Starting Monday, "The Last Word with Lawrence O'Donnell" will move into Olbermann's 8 p.m. ET/PT slot on MSNBC and Ed Schultz's "The Ed Show" will move to 10 p.m. ET/PT, NBC Universal announced.