David Letterman has the longest tenure of any late-night talk show host in U.S. television history.

NEW YORK — David Letterman’s departure from the late-night realm won’t just end an unmatched run on television. It also will close the book on an era reaching almost to the birth of TV.

During a taping of Thursday’s edition of “Late Show,” Letterman startled his audience with the news that he will step down in 2015, when his current contract with CBS expires.

He specified no end date, saying he expects his exit will be in “at least a year or so, but sometime in the not too distant future — 2015, for the love of God, (band leader) Paul (Shaffer) and I will be wrapping things up.”

What he’ll be wrapping up is three decades on the air — the longest tenure of any late-night talk show host in U.S. television history — since he launched “Late Night” at NBC in 1982.

But more than that, he’ll be ending a lineage of late-night hosts who pioneered talk and humor in the wee hours — Johnny Carson, of course, and, before him, Jack Paar and especially Steve Allen.

Ironically, they were all on NBC, the network that denied Letterman the “Tonight Show” crown he sought and, after he lost out to Jay Leno, prompted him to pitch his tent at CBS as Leno’s rival.

Referring to CBS chairman Leslie Moonves as “the man who owns this network,” Letterman told viewers Thursday, “I phoned him just before the program, and I said, ‘Leslie, it’s been great, you’ve been great, and the network has been great, but I’m retiring.”’

Along with his network, Letterman thanked “all of the people who have worked here, all of the people in the theater, all the people on the staff, everybody at home — thank you very much.

“What this means now,” he cracked, “is that Paul and I can be married.”

Since premiering with “Late Show” in 1993, Letterman, who turns 67 next week, has reigned at Broadway’s Ed Sullivan Theater, a historic venue nearly a century old that was famously home to “The Ed Sullivan Show.”

The Los Angeles-based Leno, 63, retired from “The Tonight Show” this year, clearing the way, not by his choice, for “Late Night” host Jimmy Fallon to move up to that TV institution.

In contrast to Leno, Letterman’s leave-taking appears to be his choice.

“For 21 years, David Letterman has graced our network’s air in late night with wit, gravitas and brilliance unique in the history of our medium,” Moonves said. “It’s going to be tough to say goodbye.”