Nightline's longtime executive producer, Tom Bettag, will also leave. ABC News President David Westin said he and Koppel "discussed a number of options under which he might have remained at Nightline or in some other capacity at ABC News." But, "Koppel believes this is the right time for him to leave," Westin said in a statement Thursday.
"As much as I will regret his leaving, he is firm in his conviction, and I respect his decision," Westin said.
Westin said he was optimistic that Nightline will continue with an "orderly transition."
In his statement, Westin praised Koppel's work at Nightline for the past 25 years, saying the program "represented the best of what we can achieve in reporting on the important and difficult issues of our day -- all done with the utmost intelligence and integrity." Westin did not mention who might replace Koppel on Nightline after he leaves or whether the format would change.
Still, the show's future does not appear guaranteed. Koppel and ABC News executives had hammered out a transition plan when he signed his last contract five years ago, but it blew up in 2002 when ABC's entertainment division secretly tried to poach Late Show host David Letterman from CBS to fill Nightline's slot. Letterman chose to stay, but the incident made Nightline employees question ABC's commitment to their show.
Westin has assured him that he was not being pushed out the door, Koppel, 65, told the Associated Press Thursday.
"But who knows?" Koppel said. "Maybe it was. I'm too much a reporter and a realist, and have been in this business too long, not to recognize that my salary is very high, particularly for someone who only does three days a week now."
As a direct competitor to Letterman and NBC's Tonight Show with Jay Leno, Nighline's audience has dropped from an average of 6.3 million a decade ago to 3.8 million this season, according to Nielsen Media Research.
"Maybe they feel that it's time to give somebody younger and willing to go downmarket a chance, but I'd only be speculating," Koppel told the AP. "I hope they don't go downmarket."
Koppel's decision to leave the program comes at a time when network news programs have been losing audiences to cable news channels and to other media sources such as the Internet. The announcement also follows the departures of NBC Nightly News anchor Tom Brokaw in December and CBS Evening News anchor Dan Rather earlier this month.
ABC World News Tonight anchor Peter Jennings is the only one of the top network anchors of the past 20 years who remains in the chair.
Nightline began as a series of special reports during the Iranian hostage crisis in November 1979 and was originally anchored by Frank Reynolds.
The show attracted controversy in April 2004 when Koppel devoted an edition of Nightline to reading the names and broadcasting pictures of soldiers killed in Iraq. Sinclair Broadcasting refused to carry the special Nightline edition on its seven ABC affiliate stations, deciding that the broadcast was "motivated by a political agenda designed to undermine the efforts of the United States in Iraq."
Koppel, who was embedded with U.S. troops in Iraq in 2003, denied he was opposed to the war, saying that reading the soldiers' names was a politically neutral way of honoring those who died.
Leroy Sievers, Nightline's executive producer, told the NewsHour that "we were looking for ways to humanize, to make these numbers (of those killed) into individuals."
Koppel said in an e-mailed statement that he's proud of the show's series on the criminal justice system, race relations in the United States, and its coverage of issues that do not get much attention, such as the ongoing wars in Congo.
Koppel, who has worked at the network for a total of 42 years, told the AP that a news release about his departure came out at 10:59 a.m. on Thursday, and two minutes later he got his first job offer, but he wouldn't say from whom.
"It was sort of lovely," he said. "I was disappointed that it didn't come in at 11, of course."