"Charlie Gibson is one of the most distinguished journalists on television. He is a superb broadcaster, the consummate professional, and a very familiar presence to the audience and everyone at 'World News Tonight,'" ABC News President David Westin said in a statement.
The network says Gibson will take the post on May 29 and will continue as a co-anchor of "Good Morning America" until June 30. ABC did not immediately name a replacement for Gibson on "Good Morning America."
ABC News President David Westin's announcement came less than a week after "World News Tonight" fell to last place in the evening news ratings for the first time since 2001, behind NBC and the resurgent CBS. CBS has hired NBC's Katie Couric to become its evening news anchor.
"I am humbled to accept this new assignment. I have witnessed firsthand the grace and determination of every member of the staff of 'World News Tonight.' I look forward to joining this extraordinary team and to helping the broadcast start a new chapter," Gibson said in a statement on the network's website.
Westin had appointed Vargas and Woodruff in November to replace the late Peter Jennings in ABC News' lead anchor role. He had approached Gibson then about doing the job for a short period of time before the younger anchors took over, but Gibson balked at the deadline.
Westin had said he envisioned the job being too much for one person, since he wanted the anchors to travel frequently to news sites and do separate versions of "World News Tonight" online and for West Coast audiences.
But the Vargas-Woodruff team was in place for only a month before Woodruff was injured.
Vargas, who is pregnant and goes on maternity leave later this summer, will return in the fall to co-anchor the "20/20'' newsmagazine, ABC said. Vargas said her doctors had asked her to cut back her schedule.
"I have loved every day I spent at 'World News Tonight' and have endless respect for my colleagues there," she said. "This broadcast needs someone who can give 150 percent, day in and day out. I am not in a position to give that right now and it wouldn't be fair to do any less."
ABC's announcement did not specify a role for Bob Woodruff, Vargas' co-anchor on the evening news until he was seriously wounded in a roadside bombing in Iraq on Jan. 29. Woodruff is still recovering from serious head injuries and broken bones.
In a news release, Woodruff called Gibson a mentor and friend, and said: "I look forward to contributing to his broadcast as soon as I am able."
Gibson returned to "Good Morning America" on January 18, 1999. He previously co-anchored the morning program from 1987 to 1998. In addition, Gibson was also co-anchor of "Primetime Thursday," now known as "Primetime," and was a regular substitute anchor on "World News Tonight."