The 45-year-old former Panorama presenter has a tumour growing on his pituitary gland but plans "to get on with his life".
The condition, being monitored by doctors, was found after he suffered a serious blow to the head last month.
The 45-year-old's agent said Bashir has a potentially life-threatening tumour, which was found during treatment for a head injury last month.
He was taken to hospital after suffering an injury on the set of his Nightline show for ABC News in America.
Bashir, who is back at work, says he plans "to get on with his life".
The pituitary gland is located at the base of the brain and helps to control growth.
Mr Bashir's agent, John Miles, said: "It has been a very worrying time for Martin and his family but the good news is that the specialists do not want to operate on the tumour yet.
"The doctors say they have found it at a very early stage. Obviously it's very serious and they are not unconcerned."
Mr Miles added that while the news was "very upsetting, Martin is seeing it as a blessing in disguise"
"If he hadn't had the knock to his head, he would never have known about the tumour" Mr Miles added.
Bashir lives in New York with his wife Debbie and their two children. He is due to fly to Los Angeles this week to make a programme about gang murders.
Bashir then sealed global recognition after securing the first in-depth interview with Michael Jackson in 2003.
The show sparked a court case with Jackson facing charges of child abuse - he was later cleared by a jury.