Brokaw, 74, was "diagnosed with multiple myeloma, a cancer affecting blood cells in the bone marrow, in August at the Mayo Clinic," the network said.
"His doctors are optimistic about the outcome of the treatment he is receiving, and Brokaw said he remains 'the luckiest guy I know,'" it said.
Brokaw sat in the anchor chair of the "NBC Nightly News" from 1982 until he stepped aside in 2004 to serve as a special correspondent for the network. He joined NBC as a reporter in 1966.
"Throughout the treatment, Brokaw has continued to work on NBC News projects, including a two-hour documentary on the assassination of JFK, appearances on 'TODAY,' 'Nightly News with Brian Williams,' 'Meet the Press' and MSNBC," the announcement said. "He is also contributing to NBC Sports coverage of the Winter Olympics in Sochi."
The NBC release included a personal quote from Brokaw.
"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 remain the luckiest guy I know."
Brokaw said in a 2012 interview with CNN that the death of ABC news anchor Peter Jennings from lung cancer in 2005 "really affirmed my decision" to retire.
"The line that you don't know anybody near death who says 'Gee, I wish I had spent more time at the office,'" he said. "That's kind of how I felt. I wanted to be free to do things that I care about, and not worry about whether or not it was a rating period or I have to rush back to the studio to be on air."