Gibson, 66, said he had been planning to retire at the end of 2007 but events compelled him to stay. He was named anchor following the death of Peter Jennings and the wartime injury of Bob Woodruff in 2006. He's been at ABC News for 35 years and said he plans to continue as an occasional contributor.
Sawyer's elevation means that, with Katie Couric at CBS, two of the three leading anchors for the broadcast networks will be women.
Gibson was an instant ratings hit at "World News" at a time the other networks had much younger anchors. But NBC's Brian Williams eventually passed him by and has been leading in the ratings for the past year, with "World News" a solid second.
"The program is now operating at a very accelerated, but steady, cruising speed and I think it is an opportune time for a transition -- both for the broadcast and for me," Gibson said in an e-mail to fellow ABC News staffers. "Life is dynamic; it is not static."
Sawyer will leave a hole at ABC's "Good Morning America," where she was co-host with Robin Roberts. ABC said it had no immediate announcement on what will happen on that show, which is also second in the ratings to NBC.
Gibson's biggest impact at ABC has been when he stepped into the breach during times of need.
He spent 11 years as co-host of "Good Morning America" before stepping down in 1998. But with the program imploding in the ratings, ABC News president David Westin asked him to come back and team with Sawyer. What was envisioned as a stopgap of a few months lasted until mid-2006.
After Peter Jennings died of cancer in 2005, Westin replaced him with an anchor team of Woodruff and Elizabeth Vargas. But after Woodruff was seriously hurt in a wartime injury and Vargas became pregnant, Gibson was asked to take over.
"What I value the most is that he really did bring a sense of calm and stability to a broadcast that really needed it," Westin said.
Gibson is passionate about the news but is also able to bring the right tone to a story, Westin said. The Princeton graduate has a more buttoned-down style than Couric and Williams, yet he ends the broadcast with a personal note to viewers: "I hope you had a good day."
Gibson had the first TV interview with GOP vice presidential candidate Sarah Palin in 2008, where she answered a question about insights into Russian policy by noting that you can see the country from Alaska. With Gibson as anchor, ABC News beat its broadcast rivals in the ratings on the night Barack Obama was elected president.
Westin said Gibson had approached him earlier this summer about retiring and he was asked to think it over. ABC had no desire to see him leave: "We were very happy with Charlie," Westin said.
"He was doing a terrific job."
Yet Gibson was insistent. His wife, Arlene, had recently retired as a school administrator and Gibson's only grandchild had moved with his daughter and son-in-law to Seattle over the past year.
"This has not been an easy decision to make," Gibson said. "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."
Sawyer, a 63-year-old newswoman who is a former "60 Minutes" correspondent and with Barbara Walters was a competitor for major news interviews in the 1990s, was the obvious choice, Westin said. They'd been talking about the job for a week before she called Tuesday night to tell him that she would do it, he said.
Sawyer called the job "an enormous honor."
"Diane is one of the hardest-working people I know and this new assignment is the latest achievement in an already accomplished and illustrious career," said Couric, who competed against Sawyer as a host of NBC's "Today" show. "And as I did, I'm sure that she'll quickly find that she doesn't miss that early morning alarm clock."