Tapper has been at ABC since 2003, most recently as its senior White House correspondent and author of the Political Punch blog. He's also author of "The Outpost: An Untold Story of American Valor," about the war in Afghanistan.
He's been ABC's primary substitute for the Sunday "This Week" talk show. But he was twice passed over for the top job, once when Christiane Amanpour replaced George Stephanopoulos and again when Stephanopoulos returned to replace Amanpour earlier this year.
Tapper worked at Salon.com before joining ABC, but he also had experience at CNN a decade ago, co-hosting a weekend show featuring young journalists talking about politics and pop culture.
He'll be joining CNN at a time of change, with former NBC Universal chief Jeff Zucker starting in January as head of the network.
Tapper's exit triggered changes at ABC News. Veteran Washington hand Jonathan Karl will be ABC's new chief Washington correspondent, ABC News President Ben Sherwood said.
Martha Raddatz's role will expand as the new chief global affairs correspondent. Raddatz, who won praise this fall as moderator of the vice presidential debate, will become Stephanopoulos' chief substitute on Sunday mornings.