Current founders Al Gore and Joel Hyatt said in a letter to viewers that Olbermann was let go due to a lack of "respect, openness, collegiality, and loyalty to our viewers."
Olbermann will be replaced by a frequent Countdown guest, former New York governor Eliot Spitzer.
The writing had reportedly been on the wall for Olbermann, who missed 19 out of 41 working days in January and February, including nights that Current was covering crucial Republican primaries.
Olbermann denies the Current founders' allegations, and plans to sue over his dismissal.
"It goes almost without saying that the claims against me implied in Current's statement are untrue and will be proved so in the legal actions I will be filing against them presently," he said in a statement.