Mr Carter said that doctors made the discovery during a recent liver surgery. They also discovered that the Cancer has spread to other parts of his body.
The 90-year-old statesman underwent surgery to remove a small mass in his liver, earlier this month.
He said he will reveal more details “when facts are known, possibly next week”.
Mr Carter will undergo treatment by physicians at Emory Health Care in Atlanta.
President Barack Obama had wished him “a full and fast recovery”.
He said: “Jimmy, you’re as resilient as they come, and along with the rest of America, we are rooting for you”.
According to media reports, The White said President Obama spoke to Mr Carter via telephone.
Mr Carter was the President of America from 1977-1981.
Leaving the White House in 1981, Mr Carter has remained active carrying out humanitarian efforts with his Carter Center in recent years.
He founded the centre, which focuses on human-rights efforts and political mediation, soon after he left office.