The 74-year-old Brown is receiving a short course of conventional radiotherapy for "localized prostate cancer," the statement said.
Brown's "prognosis is excellent, and there are not expected to be any significant side effects," University of California, San Francisco oncologist Eric Small said in the statement. Small is Brown's oncologist.
The radiation treatment will be completed the week of Jan. 7 nearly four weeks from now and Brown will continue to work a full schedule, the statement said.
Brown's spokesman Gil Duran declined further comment.
Prostate cancer is the most common cancer in men. More than 241,000 new cases are expected to be diagnosed in the United States this year. More than 90 percent are early stage, and nearly all men with such diagnoses survive at least five years.
Localized prostate cancer means "the tumor is still contained within the prostate," said Dr. Mark Litwin, chairman of the UCLA Department of Urology, who is not involved in Brown's care. "Of course, that's what you want because you can treat it much more effectively."
It is the governor's second bout with cancer. He underwent minor surgery in spring 2011 to remove a cancerous growth on his nose. He was put under local anesthetic and doctors removed basal cell carcinoma, a common, slow-growing form of skin cancer, from the right side of his nose.
For that cancer, Brown underwent micrographic surgery, in which a doctor can tell even before the wound is closed that all the cancerous cells have been removed.
Typical radiation treatment for an early stage prostate cancer is five days a week for four to five weeks, said Dr. Ralph de Vere White, urological oncologist and director of the University of California, Davis comprehensive cancer center in Sacramento.
Not all men who are diagnosed with the disease choose to undergo treatment, and doctors advise patients to consider the risks.
De Vere White, who is not involved in Brown's case, said given the governor's otherwise excellent health, it's an easy decision to do the radiation treatment.
"When you're as healthy as the governor is, and you are looking out at 10 years, then you go for a treatment that is going to have in excess of a 97 percent cure rate," he said. "It really should have very minimal side effects, should have minimal to no interference with his life, and kind of represents the reason why people advocate for finding this disease early."
The governor's office did not say how the prostate cancer was first detected.
Brown is the son of former two-term governor Edmund G. Brown and has spent a lifetime in politics, including terms as the secretary of state, attorney general and mayor of Oakland.
He previously was governor from 1975 to 1983, and returned to his former post in 2011 after handily beating former eBay chief executive Meg Whitman.
Brown became the state's oldest serving governor when he turned 73 a few months into office. He was its second youngest when he was first sworn into office in 1975.
The typically energetic Brown is coming off a resounding political victory in November after he persuaded Californians to support Proposition 30, a ballot measure that raised the statewide sales tax and increased income taxes for the wealthy.
The extra revenue is expected to help ease the state's budget woes and give Brown smoother sailing to pursue his agenda in 2013.