Mills was at the studio to perform with Joseph Arthur and inadvertently broke the news to the world via Twitter.

Late-night TV's longest-running host is riding off into the sunset. David Letterman announced his retirement as the host of CBS' Late Show during this afternoon's taping of tonight's broadcast. He will remain host until 2015, when the two-year contract he inked in 2013 expires.

The news was first broken via a tweet from Mike Mills, bassist of R.E.M. through their breakup in 2011. Mills was on hand as the night's musical guest, performing with singer-songwriter Joseph Arthur.

When Letterman broke the news, he mentioned calling CBS president/CEO Leslie Moonves earlier today to thank him for his support. While he assured he'll be staying with the program into 2015, he was unsure of his particular end date. After delivering the news, Letterman received a well-deserved standing ovation from the studio audience.

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Letterman's exit will provide yet another jolt to the late-night talk show landscape, which underwent a massive transformation earlier this year with Jimmy Fallon replacing longtime leader Jay Leno as the host of The Tonight Show. Seth Meyers took over for Fallon on Late Night in late February, exiting from his role with NBC's Saturday Night Live.

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Letterman has been with CBS' Late Show for the past 21 years. His retirement was confirmed via the following official statement from his representative:

"David Letterman, during a taping of tonight's Late Show, said that he informed Leslie Moonves, president/CEP of CBS Corporation, that he will step down as the host of the show in 2015, which is when his current contract expires."

Dave just announced his retirement #2015 #muchlovedave

— Mike Mills (@m_millsey) April 3, 2014

@MCNtweets 300 or so people in the audience, all with mobile phones, someone would have if not me.

— Mike Mills (@m_millsey) April 3, 2014

This story originally appeared on Billboard.com.