The 72-year-old has multiple myeloma, a cancer affecting blood cells in the bone marrow, NBC News said in a statement on Tuesday. The Mayo Clinic discovered the illness last summer.
The network statement also said Brokaw's doctors are optimistic about his treatment and encouraged by his progress since the August diagnosis.
"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," Brokaw said in the statement.
The former national anchor is currently an NBC News special correspondent. The network says he has continued to work on projects during his treatment and is contributing to NBC Sports coverage of the Winter Olympics.
Brokaw said he is grateful for the interest in his health, but he wants to keep it a private matter.
He began his career with NBC News in its Los Angeles bureau in 1966, later serving as its White House correspondent during the Watergate scandal before becoming anchor of "NBC Nightly News" in 1983.
Brian Williams replaced him as anchor in 2004.
Brokaw is the author of several books including "The Greatest Generation," his celebration of Americans who endured the Great Depression and World War II.
The Associated Press contributed to this report.