Koppel, 65, has anchored the show since its official launch in March 1980. The show grew out of a series of special reports about the Iranian hostage crisis that began the previous November.
Koppeland his "Nightline" executive producer, Tom Bettag, are expected to keep working together on news programs after leaving ABC News.
How the late-night news show will evolve following Koppel's departure remains a mystery less than two months before its debut. ABC has appointed James Goldston, who produced a British show similar to "Nightline," as the broadcast's new executive producer, and it has experimented with a multi-topic format on nights Koppel was off.
The longtime Washington-based show is expected to split time between studios there and New York, according to published reports. Several reports have also suggested Koppel will be replaced by multiple anchors; ABC News has declined to talk about its "Nightline" plans until they are complete.