NBC becomes the first of the three networks to announce a succession plan for their chief anchors. Brokaw, ABC's Peter Jennings and CBS' Dan Rather have all been on their jobs for about two decades.
Williams, 43, anchors a nightly newscast on NBC's cable stations and has been Brokaw's chief substitute on "Nightly News."
Brokaw, 62, said he wanted to continue as an anchor until after the presidential elections. But he said he wanted to get a succession plan in place for Williams, who was attracting attention from other networks.
"I am here because I couldn't walk away from this story," Brokaw said.
NBC executives said they had Williams in mind as Brokaw's successor as long ago as 1993, when he joined NBC News from CBS's New York affiliate, where he was a local anchor.
Williams said anchoring the evening news was his dream job.
"I am a 43-year-old anachronism," he said. "I am the kid in front of the TV set wondering what it was like to anchor the evening news."