Diane Sawyer, shown here in 2010, is stepping down as the anchor of ABC's "World News Tonight" at the end of the year. She will be replaced by David Muir. (Kevork Djansezian/Getty Images)

Diane Sawyer will step down as anchor of ABC News’ flagship broadcast, “World News Tonight,” at the end of the year and will be replaced by correspondent and anchor David Muir, the network said Wednesday.

“Good Morning America” and “This Week” anchor George Stephanopoulos, a top contender to replace Sawyer, will add a new title, chief anchor, and handle special reports and breaking news.

Sawyer will create original programs and conduct major interviews for the Disney-owned network, in effect succeeding former ABC anchor Barbara Walters, who retired in May, as a kind of emeritus anchor and network news personality. Sawyer, who has worked for ABC News for nearly 25 years, has been behind the evening news desk of “World News” since 2009. She succeeded Charles Gibson; Elizabeth Vargas and Bob Woodruff co-anchored before he took over. Peter Jennings hosted the show for two decades until 2005.

Muir’s elevation to the top daily broadcast wasn’t unexpected within ABC. He and Stephanopoulos have been considered the two leading candidates to replace Sawyer. But the new arrangement and Stephanopoulos’s new title are unusual; the anchors of the network’s evening newscast have typically been the anchors of breaking news reports and have always been considered the chief anchor.

Muir has anchored the newsmagazine “20/20” since 2013 as well as the weekend edition of “World News.” He’ll take over for Sawyer on Sept. 2.

In a memo to employees, ABC News chief James Goldston said: “David has made his reputation reporting from the ground on every big story of recent times, and we know that when big stories happen, wherever they happen, David will be there for us and for our audience. “I am thrilled he is taking the helm at World News.”

He said Sawyer began talking with top management at the end of last year about ending her run as anchor. “As much as she loved leading World News to new heights . . . Diane decided that now is the moment to concentrate full time on tackling big issues in new ways,” Goldston wrote.

“World News” was the top-rated daily newscast during the May “sweeps” period among adults age 25 to 54, the most important group for advertisers. It was the evening broadcast’s first win in sweeps in more than six years. NBC’s “Nightly News with Brian Williams” had been the dominant newscast until then.

Stephanopoulos has helped “GMA” beat longtime rival “Today” in the morning-news rankings for the past two years. He has also helped boost “This Week’s” ratings; it usually finishes second behind the first half of CBS’ “Face the Nation” but ahead of “Meet the Press” on NBC.

In a statement, Muir said, “On this incredibly humbling day, I think of the 12-year-old boy with a dream of being a reporter and seeing the world. I never could have imagined seeing that world with Diane Sawyer. On this day I thank her for not only setting the bar for the rest of us, but for being a trusted friend. I cannot wait to begin this new adventure with Diane and with George, whose political brain power and endless curiosity inspire me, and drive us all to be better every day.”