He called the job he's leaving "a labor of love."
Since joining ABC in 1975, he served two stints as co-host of "Good Morning America," was an anchor of the "Primetime" newsmagazine, and tackled numerous assignments as a correspondent. He had anchored "World News" since 2006.
The 66-year-old Gibson announced his intention to retire several months ago.
On Monday, his former "Good Morning America" co-host Diane Sawyer will take over anchor duties at "World News" in what is planned as a low-key transition.