Carter, 90, said doctors have found four spots of melanoma on his brain, and that he would immediately receive his first radiation treatment.
"I'm perfectly at ease with whatever comes," he said.
"They did an MRI and found that there were four spots of melanoma on my brain," Carter told reporters. "I get my first radiation treatment for the melanoma in my brain this afternoon."
The cancer in Carter's liver was already removed during surgery. He said it was likely that more cancer will be found elsewhere in his body.
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Several foreign policy crises, particularly the Iran hostage crisis, crushed his bid for re-election and Ronald Reagan was elected to the White House.
Carter is better known for his decades of humanitarian work since leaving the White House.
Carter was awarded a Nobel Peace Prize in 2002. He helped defuse nuclear tensions in the Koreas and also helped avert a US invasion of Haiti.
"I feel good. I haven't felt any weakness or debility," he said.
Carter cut short a trip to Guyana in May to observe national elections, returning to Atlanta because he was "not feeling well," the Carter Centre said at the time.
This summer Carter has been travelling the country to promote his latest book, "A Full Life: Reflections at Ninety," which was released in July.
Carter's father, brother and two sisters died of pancreatic cancer. His mother also had the disease. Carter said no cancer has been found on his pancreas so far.