"It has not been an easy decision to make," Gibson, 66, told staffers in an e-mail. "This has been my professional home for almost 35 years. And I love this news department, and all who work in it, to the depths of my soul."
ABC News confirmed Wednesday that Gibson would retire at the end of the year and Sawyer would replace him.
Gibson became the "World News Tonight" anchor in 2006, replacing co-anchors Elizabeth Vargas -- who left to have a baby -- and Bob Woodruff -- who was injured covering the war in Iraq.
"Diane Sawyer is the right person to succeed Charlie and build on what he has accomplished," the Daily News quoted ABC News President David Westin as saying in a separate e-mail to staff. "She has an outstanding and varied career in television journalism, beginning with her role as a State Department correspondent and continuing at '60 Minutes,' 'Primetime Live,' and most recently 'Good Morning America.'"