Brokaw, who was anchorman and managing editor of "NBC Nightly News" from 1982 to 2004, was diagnosed with multiple myeloma, a cancer affecting blood cells in the bone marrow, NBC News said.
The diagnosis was made in August by the Mayo Clinic in Rochester, Minn.
Brokaw, 74, said in a statement he remained "the luckiest guy I know."
"With the exceptional support of my family, medical team and friends, I am very optimistic about the future and look forward to continuing my life, my work and adventures still to come," he said.
"I am very grateful for the interest in my condition but I also hope everyone understands I wish to keep this a private matter," Brokaw added.
The South Dakota native, now an NBC News special correspondent, has worked on network projects since his diagnosis, including on a documentary about the assassination of President John F. Kennedy, and has appeared on the "Today" show, "Nightly News" and "Meet the Press."
Brokaw is the only person at NBC to host all three programs.
The former anchor has also appeared on MSNBC and is contributing to NBC Sports coverage of the Winter Olympics in Sochi, Russia.
Brokaw joined NBC News in 1966, when he was 26, working in the network's Los Angeles bureau and anchoring the local 11 p.m. news. He became the network's White House correspondent in 1973, covering the Watergate scandal and anchoring the "Nightly News" Saturday editions.
He was a host of "Today" from 1976 until 1982, when he started co-anchoring "Nightly News" from New York with Roger Mudd in Washington.
The network dropped the co-anchor format the following year and Brokaw became the solo anchor Sept. 5, 1983.
He was replaced by Brian Williams in 2004.
Brokaw, who moderated "Meet the Press" for part of 2008, is also an author of several books, including "The Greatest Generation," about Americans who grew up in the Great Depression and then went on to fight in World War II or help the war effort on the home front.